My prejudice is for independent units so if one fails you don't lose the other. For me, most important instrument is depth finder. On the Chesapeake Bay, except in a storm (I usually don't travel at nite), with just a compass I can easily get from one place to another,(We have so many markers and buoys that you can jump between them readily), altho GPS makes it even easier on a longer leg. I have a chart plotter on the bridge and a plain vanilla GPS position readout in the cabin (that one is very old and needs to be replaced with the cheapest thing available).
There have been a number of posts on how to save lots of money by hooking a cheap GPS to a cheap computer and winding up with an excellent large screen display - search for those posts. Nice idea for the cabin.
Meanwhile, if your compass is working well there is no need to replace it. It doesn't require power and is a fabulous backup for GPS direction indicators. There are many links on the web on how to compensate the compass. A quick check would be to compare it to the direction info you get on the GPS (which should be set to "magnetic", not "true" for a comparison).

Joel, I was leaning towards separates, just for ease and not having to move them around. In the Great Lakes, we get some pretty thick fog and that's when it's really nice to have a GPS on board, you can go many miles without seeing bouys. Also with gps's you can mark underwater structure for fishing and be able to find it later on easily. Will check out the other posts too. Compasses are working great, just in the fog or thick haze I really appreciate the new electronics. Norm, Yeah, appreciate your info on parts, ect. As far as the compasses and correcting them is concerned, they both seem to be quite close to on the money, amazingly enough, worked fabulous and true coming around the mitt, haven't checked in my lake yet (I cracked a brewski once I 'smelled home' too!) but will do so soon (I've had electrical breakdowns in the past on other boats) to make sure the old way to get home is still an option. Laptops seem to be interconnectable with most applications/hardware these days and I believe are going to be more and more commonly seen on boats, just so you don't have to have all those separate units onboard. Jack