Only you can make that decision by comparing specs and price and flexibility.
You need to look at Apple's website to make comparisons.
Also, without knowing your budget or how much you're willing to spend, it's hard to advise.
Desktop Macs are not mobile, but they are, generally, more powerful with better specs than their laptop counterparts.
Desktop Macs are, generally, less expensive than for comparable features in a laptop model like the MacBook Pro.
For Mac Laptops, I would stick with the MacBook Pro Line. IMO, the MacBook Air line isn't quite their yet.
FYI, next week, Apple is announce new laptop and desktop computer line at the WWDC conference.
So, you may want to wait to see what's new and becoming available before you consider what Apple has on their site, now.
They have larger screens VS screen sizes on laptops.
The larger MacBook Pros have 15 inch and 17 inch screens, but you pay quite a bit more of a premium for the extra screen real estate than you do with a desktop Mac, like the iMac.
If you plan to watch a lot of video content, the larger screens of a desktop Mac , like the iMac, maybe more desirable to you.
If you're continuing on to college, then you need to weigh whether you need computing mobility.Not having mobility with a desktop computer means using other means and devices for taking/recording class notes or assignments.
If using a digital device, it needs to be compatible with Macs to allow you to sync you data to your Mac if it's not of the laptop variety. Something like the iPhone or iPod Touch or iPad or other Mac compatible device would be needed if you plan on purchasing a Desktop iMac model.
Another option for you if budget is a concern, Apple has a part of their site where they have refurbished models that are discounted. These computers undergo the same stringent standards Apple uses on their new computers and offer the same AppleCare warranties.
Also another avenue , is to look into authorized Apple resellers for slightly older models of the Macs you are considering.
I purchased two 2010 iMacs (one 21.5 inch and the other a 27 inch model) this way as I got two fully decked out iMacs for slightly under what the base model iMacs go for. Plus I was able to obtain a 6 month warranty that I can extend out, if I desire, to 1 year through the reseller I went through. Thought this was a great deal for me.
This could be another way of saving some $$$ and getting a good quality Mac.
Good Luck on your decision and purchase!