I'd say go with the ab-roller/ab wheel type deal. Other exercises you can pretty much mimic without the machine (such as the ab rocker's motion, etc), but the wheel is something that requires at least a basic tool. I've never used one myself, but I've seen a few of the other hardcore workout-a-holics at my gym using the wheel religiously. Just be sure that when you do it your feet are on the ground as opposed to up in the air (i.e. don't keep your knees locked at 90 degrees so that on the way down your feet lift up in the air; keep your feet in contact with the ground) as this will help reduce lower back strain.
I personally stick to the age-old (and possibly outdated) 25 minute ab routine every other day, but that's just me. If I bought the wheel and it seemed to do just as much as the exercises I do now, then I wouldn't have a problem switching.
You might also want to try exercises other than just sit-ups or crunches. You can do leg lifts and other exercises that place much less strain on your neck. Hope it helps.