Basically larger curves are generally better (within constraints, obviously, of layout size), even if most of your equipment is 027/semi scale, they look better on larger curves, sharper curves can be fun in places like industrial areas where you have boxcars or whatnot pushed around by small switchers. For a crossover, you may want to use something like O72,even if you use let's say O42 or O54 elsewhere because it will allow a gentler transition between the two loops, and O31 is pretty sharp and while it will allow the tracks to be closer together, it also will make for a crossover where engines and cars are more likely to derail and will require a lot slower speeds to make it through. If O72 won't fit even an O42 or O54 would be better than O31 in that case.
As far as the type of track, that comes down to what you want out of it and things like budget. Tinplate track is cheap and plentiful out there, and by far is the cheapest way to build a layout. Almost everything will run on it IME (given how tall the track is), it lets the old deep flange semi scale trains to work, will handle anything. Downside is it isn't very realistic (though it can be made better by putting extra wood ties on it, ballasting it and painting the rails). Menards is now making tubular track, RMT was, and there are still a lot of old switches and stuff out there, plus Ross can be mated to tubular track (Ross Switches), so you have a lot of choices if you go that way.
Fasttrack and real trax are kind of the spiritial descendent of traditional tubular, they are track built into roadbad, can be put together easily for floor layouts and the like, are relatively simple. They can be made to look better by ballasting them or doing things like weathering and staining the plastic roadbed, but they aren't particularly prototypical looking and (to me) are relatively expensive for what they give (and key word there, to me, others like fastrrack and real trax, works well for them, nothing wrong with them). Gargraves track uses tubular rails but has wood ties and is a bit more prototypical looking (Ross makes sectional track that is basically gargraves rails on their ties). Gargraves is a pretty good compromise on price IMO, it typically is closer to the price of tubular when new then the more scale like track systems IME. Gargraves also can run a lot of equipment more scale size track can't, because it has relatively tall track. Atlas 20th century and MTH Scale Trax are the ones the most prototypical looking, if you are looking for track that looks more like 'the real thing' both of these are the top end IMO. For scale modellers these tend to be the cats meow, but that again comes down to preference, some people like Atlas and think it looks better, scale trax to others i better looking...both are at the high end of price range, and tend to be more limited in selection then let's say gargraves track or ross for switches, availability at times has been a problem for both systems.
In the end comes down to a tight budget, tubular will be cheaper all around, you can get it really cheap especially on the used market and the switches are not particularly expensive comparitively.....though you also might find Gargraves doesn't cost all that much more and I am going to use it with Ross switches because it looks good for what I want yet won't break the bank for what I have planned. The thing about tubular (if that is what you have) is even if you have a lot of it, it likely won't sell for all that much, it isn't a collectors item and you see all the time people practically giving it away just to get rid of it IME, so you likely won't be able to get much for it to use towards new track.
