Planning Strength, speed, stability..

OK its been 3 months since I last posted about building another router,, I have the linear bearings now to start the build and have been trying to get the best ideas that I can find on the internet..
I am going to need one more HIWIN linear rail thats 58" long I have 1 rail and 4 bearing that i picked up on ebay and they are like new but the new piece of 20 mm HIWIN rail from a dealer is more than I paid fo 1 rail and 4 bearing so the 58 inch rail will give me approx 48" travel for the Y and I have the linear bearings for the X and Z so the travels will be 48" x 25" with 6" of Z travel ,, Money has been tight so I have just been picking up a few pieces at a time until I get all the parts and design together but now that I have bearings that helps because I know the travels now .. I will be using 2" square tubing for most of the parts of the main frame and 2x6 square tube for the gantry and 1/4 in steel sides for the gantry ..
One more question about the gantry bearings , I have noticed that some routers have the x axis bearings on the top of the side frame rails and some are on the side of the frame and I have even seen some with the bearings under the table top to keep them cleaner..Which settup would be more stable?I can see the benefit of the top and outside of the rail but which would be more stable? On linear bearings where would the load be better distributed?Also I have seen some people using belts insted of acme or ballscrews what is your opinion on that choice?I have acme screws on the wood router now they work pretty good but the delron nut is where the wear comes into play.. I do like the idea of a double drive on the gantry to keep any binding from coming into play..
Hope this post wasnt too long but there is soo many items to consider and I am not in anyway an engineer in machine design as you can tell from all of the (some may think simple ) questions but I did learn quite a bit from building the last router from MDF..
Thanks for the info guys
Gary