After having been in the modular world for around 10 years, I can only say that every format has its own pros and cons..
5U usually is quite big to clearly see your connections and still put your hands in the middle to happily knob away... Something that the 3U/Eurorack format sometimes misses. Also in the years I started to develop a "hate" for minijacks, so
I am now in the direction of building a mixed 5U system.. Got a portable anvil case from here (thanks again Damir!), plus a
couple 5u modules, one from a .com and the other one is a Encore.. I am quite "fed up" of the "classic" signal flow that
you can find also in non-modular synthesizer (VCO-VCF-VCA and the like), so nowadays I am more keen in finding signal
processing modules, like dividers, clocks, filters, etc... Plus anyways I would like to have a very good (and I mean VERY
GOOD) VCO. But I am still undecided on that...
I have built some modules based on the designs of Ken Stone and Ian Fritz (Synthacon Filter, Super Psycho LFO and the
ChaQuO), you can nowadays find complete kits (PCB+parts+panels) from Elby Designs in Australia...
So self building is another interesting, related subject...
In general: try as many different systems you can, travel the world to see them, read forums, listen to recordings...
And get your own idea
a.