You'll find lots of threads discussing the pros and cons of RFB vs TMO etc.
RFB and TMO adjust the gameplay (plus some bug fixes and graphic enhancements) whereas RSRD adjusts the campaign layer etc to provide a more historical campaign (i.e. ships and TFs in the right place at the right time).
I'm running TMO with RSRD and couldn't be happier. The enemy AI can be a challenge (some say too much, LOL). Others prefer RFB with RSRD. I doubt you'll be disappointed with either. If you are an experienced SH3/GWX player then I'd suggest the sooner you move from SH4 Stock the better.
