I haven't tried the Speed 3, but I have run in the Speed 2 (got to be at least 600k by now and still basically good as new) and the Pro 3 (just 2 runs). The Speed 2s are really quite bouncy and feel great at faster paces but feel a tiny bit awkward at easy pace. They're fairly firm (but still fine for a half) and not super stable but you could probably run in them every day. The Pro 3s are a lot more cushioned and feels much more like a normal shoe. They feel great for long runs and threshold work but I'm not sure if I'd enjoy them as much as the Speed 2s on something like 8x400m. I've also heard they're not as durable and the Speed 3s might be similar.
Never pay full price for a pair of shoes (unless you went to a store and got on a treadmill to get fitted, in which case I think it's fair to buy one pair from the store in return for the fitting). I got the Pro 3s for £126 by searching around. My original pair of Speed 2s were £100 and you can still find them for that price if you look hard. If your shoe size is extreme you can often find them for £80 which is a no-brainer price imo. If your feet are narrow you could consider a womens size if it's cheaper.
If it were my money I'd try to snag a pair of Speed 2s while you can