Unfortunately, the two styles of play almost make it impossible to build a single ship taht is easilly convertable from one format to another. Big Gun ships move more slowly and typically have smaller rudders. They also have much smaller motors and much less pumping capacity. On the other hand, they do tend to take a lot less damage. (Or a lot all at once, followed by an almost immediate sink). Fast gun ships move much quicker, needing larger props, rudders, and drive systems. Fast gun ships spend a large amount of their weight and space with batteries to drive those larger motors. The BB cannons for fast gun ships are much more compact than many big gun weapons. Also, unless you have some behemoth, there simply isn't enough weight to go around to arm a fast gun ship with big gun cannons, and there isn't really space in a big gun ship for the large power needs of a fast gun ship.
You might be able to get away with building one hull, but having two completely seperate internal systems that are semi-modular. However, some compromises would have to be made for both formats. If you're really into big gun format, you might just want to concentrate on that. ***Alert! Author's bias ahead!*** However, I found that big gun ships were more complex, especially to a first time builder without support from local experienced battlers and were much more frustrating to get running. On the other hand, with folks in UT and ID (not to mention yours truly here in Reno) a fast gun ship would benefit greatly from local experience. A good starter ship, like a 3 battle unit cruiser, isn't really all that daunting a task given time and some outside help/advice along the way. But, like I said, if big guns is what you want to do, by all means, boats is boats! Just make sure you get in touch with the big guns folks for their help as well. 
Let me know if you are able to come down for any of our events here in Reno. With a week's notice, we can battle/build on any thursday or friday afternoon.