Using the MRT and BTS you can explore a lot of Bangkok without sitting in time wasting traffic.
Although the BTS is bigger and has better A/C than the MRT, I prefer the latter because it has 1, 3 and 30 day unlimited monthly passes and you get to wait for it underground where the temperature is much cooler.
The BTS and MRT interchange at two stops: Mo Chit and Asoke/Sukhumvit.
Like the BTS, the MRT does get very crowded during peak hours and I would recommend avoiding Sukhumvit and stops adjacent to it during this time.