With both these species you are also going to want to consider housing costs.
When I was your age I really wanted one of those little niles too

But, looking back I was nowhere near ready to handle an animal who would eventually get close to 6 ft long, tend to be aggressive, and actually capable of causing serious injury. You'd have to basically be able to dedicate a room, or have a large out door pen to do the animal justice as an adult. You'd also have to have a fair amount of stability. For example, if going away to college is on the horizon, what would become of your now quite large, and difficult to place monitor? Then think of the logistics of feeding such a large lizard, cleaning up after it and heating and lighting its enclosure.
Uros in petstores are almost always imports. Most are improperly cared for and stressed. This stress and poor care often causes parasites to get out of control, so be sure to budget in veterinary care. Think 60 gallon minimum tank size for most adults. Also start pricing lighting. Uros need UV and lots of heat. You're going to want to have highs in the 120's (you'll need thermometers to monitor this) and lows in the 80's. When I caught my chuckwalla (similar heating / lighting needs to uros), I ended up paying close to $200 for all the lighting and housing supplies, not including the tank!
If you've got the budget, I'd go with the uro over the nile. But, I think that it would be worth buying a uro from a breeder. Vet bills add up fast! And, if you end up having to tube feed him and administer medications, you're going to have to invest a lot of time and money. Here's a uro carsheet
http://www.deerfernfarms.com/Uromastyx_Care.htm it's a "must read" if you want to do justice to the animal. Having a good set-up before you bring your uro home is essential to getting them acclimated successfully so be sure to get the supplies before the lizard.
-Alice