I can only speak from my experience and I'm sure others will have different opinions but here goes.....
For 2005 and older (I believe 2006 was the first year for the pickle fork) you're going to find the X2 will more ski oriented. More of a no nonsense wakeboard boat that has a very good slalom wake too. The MariStar is going to be more family oriented with more of a focus on ride and comfort. Still great wakes and handling just a little dryer and smoother ride. My wife has loved all our MariStars, not so much with the 03 X2 we had for a year. You'll want to sit in both and take a ride with everyone in the both boats if possible to see which one suits you best.
This is a loaded question that keeps hitting the posts. There are those that wouldn't have anything but an MCX. I've been fine with the RTP. If you can find a boat with the MCX I see it as a plus but would be more concerned with condition and price. I've had no complaints with the RTP but others may disagree.
Depending on how you ski and at what line lengths you will see a difference. The X2 is going to be a better skier but to what degree will depend on what kind a skier you are. The MariStar is more of a recreational skier.
In the 20' range you’ve pretty much covered the field. If you're looking to go larger there are other models like the 230.
If you're surfing and wakeboarding you'll want a tower, ballast and Perfect Pass. If you like to listen to music while you ride tower speakers are a must. A stereo that has been upgraded to support MP3 format is also a plus. My boat has a heater and shower that we use in the spring and fall that are nice to have but not essential. If you're going to pull distances a tandem axel trailer is nice to have.