As for the regs... not really. Most mouth pieces for the average stuff (scubapro, etc) is about the same. Some have different features and stuff, some require you to suck air alittle more then others, etc...

I would still recommend to try out a few types first... or just go with a basic unit from a dive shop expecting to upgrade after a short period of time.

As for what I wear under my wet suit... its the same as what i wear under my kilt! LOL... nah, shorts... swim trunks, etc... Usually on a dive boat and just put the suit over what i'm currently wearing at the time.

"One should not increase, beyond what is necessary, the number of entities required to explain anything"
William of Ockham (1285-1349)