I have the opportunity to purchase a AR15. I've got it narrowed down to two models. One has a flat top and the other has the carry handle.
I'm not sure if I will want a scope on my gun. Probably will after I start shooting it.
Here's the problem, the flat top is $215 cheaper than the one with the carry handle.

Is it a good idea to mount a scope to the carry handle if I go with that model?

Or will I be better off getting the flat top model and adding a good scope to it for the extra money saved?

A scope mounted to the carry handle will likely be far too high to be effective, unless you get a serious cheek pad. A lot of guys do it, but they typically have red dots or reflex sights because eye relief and exit pupil aren't an issue - the bullet is going to go where that dot is regardless of where their eye is.

If you do get the one with the handle, mount a scope, and don't have a good cheek weld, you'll be stuck using a red dot because you won't be able to utilize a magnifying scope correctly. The offset between where the scope should be to line up with your eye is just too far off.

If you want a scope AND a red dot or reflex, Warne makes the RAMP system for the AR so you can do both:
Warne RAMP30 (30mm scopes)
Warne RAMP1 (1 inch scopes)

I personally don't see the added value in the carry handle. Get a flat top AR (they are designed for magnified optics), put a scope on it, and if you decide you want a carry handle, get a used one on Gunbroker for $75 and you can swap them out. You save $160 bucks and get both.

Ditto on the ramp system!!!! You can do so much more without a handle!! Good luck and post some pic's when your done with it!!!

Flat-top if you intend to shoot "targets" beyond 100 yards with any accuracy (MOA).

+1 for the flat-top...I have both and the flat tops get shot...the two with carry handles sit in the safe.

With all the good advice, Flat Top it is. I'm thinking about putting the Nikon ProStaff or Buckmaster 3x9x40 scope on it. It that a good choice?

I have a couple Nikon Pro-Staff scopes...for the money they are hard to beat in my opinion.

AR-15's are soft shooting guns.

I'd still look for a lifetime warranty !

For the money, I think a Minox ZV3 would give you better glass, and comes with a lifetime replacement warranty. For a bit more, the Vixen 4-12x40 is even better, and that is covered for the life of the product from everything short of the apocalypse.

But that's not to say the Buckmaster is bad, they certainly have a good track record. Any 3-9x40 in the ~$200 range is a good choice. My thinking lately is that the mass produced names are kind of getting comfortable, and the lesser known names are quickly catching up and surpassing them in quality and value.

Just my $.02.

How are the Minox zv3 and Vixen in low light conditions? Are they same, or better than the Nikons?