Easiest way to learn your towers and their upgrade path is by becoming a Sprint employee. Fastest way is by becoming an S4GRU sponsor. Maps are available to sponsors that give out tower information. That's all I'll say.
Clearwire spectrum is 2.5, Nextel is 800. Your phone can take advantage of the 800SMR, which increases voice coverage. 800LTE is not available for every phone. IIRC, your phone only does LTE on the 1900 band, meaning you wouldn't benefit from 800LTE except where it brings users off 1900, which means faster speeds for you.
If you're that concerned about 4G coverage indoors, you'll need a phone or carrier that has a low-frequency LTE with a network to match. 800LTE hasn't really launched anywhere, but it'll penetrate walls better once it has.
AT&T is 700 & 1900, same as Verizon. It's really the 700 band that sets the networks apart. Once Sprint gets their 800 fired up, any 800LTE device will get solid 4G indoors.
800LTE devices on Sprint so far are the Zing, the MiFi 500, the HTC One Max, the Galaxy Mega, the Galaxy S4 Mini, the LG G2, the iPhone 5c, the iPhone 5S, and the Nexus 5. Look for a Galaxy S4 and Note 3 re-release marketed as "Spark-compatible".
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