Verizon's LTE is
considerably faster that WiMax from either Clear or Sprint.
However, coverage is far more important than speed. Just because one carrier is "generally better" inside buildings doesn't mean it will be any better inside YOUR building. The only way to find out for sure is to buy a modem from each carrier, try it out, and return the less-effective one within the no-penalty trial period.
Even if Verizon or Clear works better in one part of your building, there's no guarantee that it will work better in YOUR part of the building.
As for pulling the maximum signal, try going closer to a window, if you can. If not, you might want to consider lifting the modem as high as possible by way of a USB extension cable. Failing that, 4G-banded antennas are around $30 - $50 -- just make sure you get the right one for your particular application (LTE or WiMax).
Good luck!