The d5000 has fewer lens options with its lack of built in AF motor. Many great lens' are rendered obsolete without AF, such as the excellent Nikkor 50mm f/1.8. You can still use all the Nikon lens, but unless the AF motors built into the lens, you have no auto-focus.

If you only plan on purchasing the one lens, then it doesn't make much difference, but if you want future upgradability then the Nikon is much more limited.

The T2i also has the higher resolution sensor, plus you already have the speedlite. The speedlite would actually work on the Nikon, though only in manual mode (pretty sure anyway, someone correct me if im wrong).

You mentioned taking photos indoors, in which case, the telephoto zooms you linked are pretty slow, I'd reccomend looking into a 70-200 f/2.8 or f/4 at most. Canon/Nikons f/2.8 is pricey though at about $1.9k, but Tamron makes one at ~$750, or, Canon makes their 70-200 f/4 L for about the same price.

Best buy is almost the last place I'd look for gear, KEH.com BHPHoto.com and Adorama.com are 3 places regularly frequented by most of us here, all 3 run a very good business with competitive pricing.