I just started riding, about a month ago. My wife and I are sharing a 250 Ninja to ride. From the research that I did, the ninja and the blast are pretty close on power. The ninja is cheaper though, and has a much better reliability history. It's going to be a lot easier to find a used ninja if you go that path, and a new one is a good bit cheaper. Kawasaki also has a much better financing plan on new bikes. The ninja is around $1500 cheaper new and around 60 pounds lighter.
The seat is really low on the blast, but the ninja is short as well. My wife is 5'6" and I'm 5'11". We both fit fine on it. My wife can get both feet flat on the ground at a stop, and the bike doesn't cramp me at all. I put 200+ miles on the ninja one day, and my butt was the only sore spot, and it wasn't that bad. Since I've gotten used to it, I've had no problems with comfort. I've read that the blast also vibrates a lot worse than the ninja. Also, the belt drive on the blast is supposed to make shifting tougher than the chain on the ninja.
Also the ninja holds almost twice as much gas. Lot more time between fill ups.
Here's some stats from a 2004 buyers guide.
Blast:
-MSRP....................$4595
-Engine...................ohv Single (the single causes the extra vibration)
-Tranny...................5 speed
-Final Drive..............Belt
-Front Brake............Disk
-Rear Brake.............Disk
-Seat height.............27.5/25.5"(I've sat on these and the seat pushed me
really far up onto the gas tank.)
-Fuel Capacity...........2.8 gallons
-Dry weight...............360 pounds (dry is without fluids, gas, and battery)
250 Kawasaki Ninja:
-MSRP.......................$2999
-Engine......................dohc Twin
-Tranny..................... 6 Speed
-Final Drive.................Chain
-Front Brake................Disk
-Rear Brake................Disk
-Seat height................29.3" (very comfy seat)
-Fuel Capacity.............4.8 gallons
-Dry weight.................304 pounds.
Also I don't know the mileage on a blast, but we're getting around 65 miles per gallon on my Ninja, and we're just out of our break-in period. I'm sure the blast get's good mileage as well.
The ninja is also liquid cooled (like your car, with a fan and radiator) which is great for slow speeds and stop and go. I think the blast is air cooled, but I'm not sure really.
I don't have anything against the blast or buell, I just giving you the numbers I used to choose between the two. I was considering a blast, 250 ninja, 500 ninja, and a gs500f. Hope this helps and good luck.
