While I have had a lot of time on the Contra 23.5m, I have had only one 4-hr session on the SPEED 17m - and while the windspeed during that session covered the whole range of the kite, my opinion may change for the better or worse after I have a little more time in. If my opinion changes at all, I will repost here.
SPEED (no contest) - SPEED 17m: 8lbs, Contra 23.5m: 12lbs
Setup / Packdown:
SPEED (no contest). MUCH shorter setup time, even if you have to preinflate a little. Packdown time is even less, just unzip the vents and roll it up. If you only have an hour to try to get a session, the SPEED will give you 30min of water time, the Contra might not give you anything.
SPEED. Lighter, more compact, better bag design - the bag for the SPEED allows for carrying 2 boards, the Contra bag can only do one (and it has a tendency to scrape the back of your legs).
SPEED. Try pumping up in -10F windchills, it can be done but it really sucks. Also, the SPEED 17m will stay airborne in winds that the Contra won't, so you can landboard with it while waiting for the wind to come up enough for a water session.
Ease of use (non-flying, i.e.: setup, etc.):
SPEED (once you learn how it works). If you're only familiar with LEI's then the Contra is a standard LEI and there's nothing new to learn, but the SPEED is easier to set up and get going. Self-launch with the SPEED is WAY easier.
Ease of use (in flight):
Contra. It flies like an LEI, not prone to backstall (much), very straightforward. The SPEED requires a different touch so you'll need good flying skills going in, and some time getting used to it. If you don't want to have to adjust your flying style, or you're low on kite skills, you won't like the SPEED.
SPEED, but not by much. Depending on the bridle settings, it will turn as slow as (or slower than) the Contra, or much faster. But on the faster settings the kite is more prone to backstall and riding style will have to be adjusted accordingly. On the stock settings, it turns slightly faster than the Contra 23.5m, except at the edge when first getting started.
Haven't tried it yet on the SPEED, but it seems easier than the Contra - getting a 23.5m kite back in the air in light wind is a real challenge, even with RECON. In addition, the SPEED stayed solidly in the air in winds where the Contra would have been questionable at best, so you're not as likely to need to relaunch the SPEED.
SPEED. Jumps felt bigger on the high end, and during my first session I wasn't even aware of how light the wind had gotten near the end - still on my small board and still going upwind. Absolute low-end isn't increased a whole lot, but it is slightly better.
SPEED. The Contra doesn't have a spinning leash, and landing on the leash with the SPEED will park the kite on the trailing edge - Contra usually lands LE down and seems to have more pull in that position than the SPEED has when parked.
Contra. RECON has a release on the bar and a QR on the chickenloop. The SPEED only has a QR on the chickenloop, so I will be putting a Slingshot Surefire spreader bar on my harness to provide a backup.
Windrange - top end:
Too close to call, so I'd say they're basically the same...
Windrange - bottom end:
SPEED. Near the end of my first session the wind was avg 9mph, gusts 12mph lulls 7mph, and I was still on my small board and still going upwind (although not as far as earlier). The Contra would barely get me started in those conditions, and ONLY with my light-wind board.
[UPDATE:] The low-end really depends on whether you're already up and going or if you're trying to get started - when trying to get started, the low-end is only slightly better on the SPEED (maybe 1mph or so). But once you're up and going, it can tolerate some pretty low lulls without completely losing it like the Contra might.
The Contra will go fast for its size but starts pulling downwind quite a bit sooner than the SPEED.
SPEED (no contest). I was going upwind as far with my short board and the SPEED as I usually go with the Contra and my long board.
Both of these are big kites, best suited to heavier riders in light wind - neither one turns as fast as a 14m LEI. If turning speed is most important to you, get a really big board and stick with a 20m LEI or smaller (or maybe the 13m SPEED). But unless you're really stuck on LEI's or your kite skills still need development, get the SPEED 17m - with few exceptions, it is the better light-wind kite. As with anything different, it will take a little while to get the feel of it, but IMO the benefits should more than pay back any learning time.
NEW: Power delivery:
I'm leaning toward the SPEED on this but need more time to be sure. The power delivery on the SPEED seems much smoother than the Contra, this has a noticable impact in two areas: gusts, and waterstarts in light conditions.
I didn't really notice any gusts until I got back on shore and a friend pointed at another guy on an LEI, the LEI was doing the surge/flex thing even though the guy was basically going in a straight line and didn't appear to be pumping the bar that much. I think maybe the foil's "soft" shape tends to soak up small/spiky gusts rather than twitching around like a rigid kite, although the larger/sustained gusts definitely are translated into more power - but more smoothly than the Contra. The wind chart shows it wasn't that gusty anyway, so it's hard to be 100% sure on this.
Waterstarts in light wind:
For waterstarts in light wind, the Contra has a bigger surge on the downstroke, and then loses a lot on the upstroke and if you're not sheeted out it can easily stall (in light wind). The SPEED had nearly constant power on the water starts, especially on +1 hard steering - the first turn is pretty sluggish, but the downstroke is nice steady pull, and that same pull continues on the upstroke. I think this is why I could still get going on my small board, the steady pull gradually got me up on plane, while the Contra would get me planing then bog down, then plane and bog down again. I also didn't have to work the SPEED as hard, the Contra has to be dived almost straight down to (hopefully) get enough board speed to coast through the upturn, while the SPEED only had to go halfway down or so because it was constant power the whole time - and increased as the board speed increased.
I can only attribute this to the weight of the kite and the efficiency of the profile - the Contra generates a bigger spike on the downturn because gravity is helping, but then you lose it on the upswing because what gravity giveth, it soon taketh away - without a big board to help you coast through that first upswing, you're going nowhere.