For what it's worth.. I'm not up the standards of most 'fine scale' modellers.. but my first (and only so far) dabble in 3mm scale was in 14.2mm. I built a working layout, (handling track using code 60 BH on pcbs sleepers), a number of society kits using society wheelsets. My motive power was a Hallig motor bogie driving a scratch built Y6 body. All great fun. (http://mogtrains.blogspot.co.uk/2009/10/elston-road-coming-along.html)
I really like 3mm. If I model British standard gauge again, it will be my first stop.
My only stumbling block was building a loco - but seeing as I haven't built one in any scale (not counting white metal bodies on proprietary chassis), that's not too surprising. I just about managed to get a working 060 chassis running.
I did think about trying 12mm gauge next - the main reason being that it strikes me that it's easier to get the wheels etc. Also the society now provide 12mm gauge SQ wheelsets, which sound a lot easier that faffing about with the 'scale' wheels for a bodger like me.
Don't be put off by 'TT' - I'm pretty sure that John Sutton's Southwell Central is 12mm gauge and it looks very fine!
The Society has tons of stuff available, but I have to say that as a newcomer and not one of the old boys, I have found it very confusing to work out what I need, and where to order it from.. that's just me - compared to the 2mm FS society for example, the 'web presence' is scant.
Cannot comment on 13.5mm except that my understanding is that it is the "EM' of 3mm. There is track available, and I think, the SQ wheels too.
Maybe it depends on what you want to build? Do you want a fine scale model? Do you want to build locos, and if so, what sort? etc..