T's can be easier to buy, and are also good if you want an extra (or you have no other) fill point you you system. The T would be at one of the higher points of the system (just inline in some tube), so you could fill you system "easily" water would only flow from A to B, rather than A to [B and C], and C would just be a place to put water, or additives in as required.
Some peopl you T's to split flow, the only reason I would is if I had a particularly difficult time buying Y's locally (I'm not big on odering plastic fittings... seems wierd to me - I'd probably just make a simple one out of a peice of acrylic before odering somethign like that.... shrug - I just can't imagine a little package with one Y fitting in it coming in the mail...)
By metal clips do you mean hose clamps? If so, if you don't feel safe with the way you put your tubing on you might wanna use clamps
(ie: if you used 1/2OD fittings and 1/2ID tubing, you probably want clamps - if you used 5/8 OD fittings, and 1/2 ID tube, you're prolly ok - but dont forget to test). My favourite clamps are the one peice metal ones that use a special "unclamp" tool. The standard screw tight ones are ok, and some people use various types of zip-ties.