Alright...so my road bike has a horizontal top tube measurement of 518mm. I'm trying to get myself a Surly Long Haul Trucker. The 46cm frame has a TT of 508.3mm and the 50cm frame has a TT of 525.1mm. Which would be a better fit?
Either one as long as you can get the reach the same with the right stem (without using one that is too long or too short), and get the seat in the right place. Keep in mind that a smaller frame may or may not allow you enough adjustment room to get your saddle as far back as you want it (because they tend to have steeper seat tubes), so check that. The other thing is that it can depend on how high you want your handlebars. The smaller the frame, the less chance you can raise those bars high enough if you like them higher. You might not even be able to get them as high as the saddle if you want that. Personally, I would say the largest frame that fits you for a touring bike rather than the smallest one.