I also have a '98 speedster like in the picture.
Light and very fast boat. Excellent for watersports. The retractable pole is very useful. Twin 787 are very fast. Lots of space to put your stuff, nice sporty look.
The challenger is heavier being a 18' has wether twin 787 or twin 717. I think the challenger is more "sunday cruise" oriented than the speedster. Look is very nice. If you want to save money, get the challenger if it has twin 717. Twin 787 is very hard on gaz when you beat them, but you get a hole lotta fun. I can go trough 3/4 of my tank in the speedster in 3-4 hours. Worse I did was 1/4 of a tank in 30 minutes.
Then again, when you don't want to spend on fuel, you don't buy a jetboat!
2 strokes are hard on gaz, smells, but they've got 1 big advantage. There's a LOT of parts available, and those parts are very cheap. Brand new 787 for 1400$, try that with a 4tec.