You can't convert a Boost Mobile Phone to Straight talk because these two phones run off of different types networks. Boost Mobile runs off Sprint Network, and Straighttalk runs off of Verizon. This means they get essentially the same coverage, and signal quality (depending on the phone model)
The biggest factor with no contract phones is determining which networks they run on. (e.g. Verizon, Sprint) the network is essential to coverage and quality.
I am a big proponent of reviews. Use the link below to see reviews of the different companies and plans, and determine the network. This should give you a good idea of which one to go with.
Once you have determined which company you would like to go with, maybe purchase a low cost phone from ebay to test it out and see if you like the coverage and quality. This way you have made a little investment to check out your research. If you like the coverage and quality, then get a much better phone but use the same network.