well...

a T3/T4 is mainly geared towards engines under 2.5 Liters... they mate the big compressor wheel (and housnig) to a small exhaust side. The T3 is the only way to get an exhaust housing small enough to efficiently spool a turbine yet make decent power on a 2.5 liter engine. Plus its convenient for import and even domestic tuners who are lookin to increase the efficiency of their small-displacment engines. Mainly because its a relativly universal turbo and has many different combinations to suit many needs and also is available w/ an internal waste gate... an option that can save hundreds of dollars, packaging issues and space under the hood as well as limit the complexity of the system.

Straight T4's (T04B/E/S/R, etc) are for larger engines, that are over 2.5 Liters in displacment... or where >2.5liters feed one turbo,, like on a V8. a 5.0 Liter V8 will do WELL with T3's or T3/T4's... simply strapping on a pair of T3's from the Turbo coupe's will make that thing fly, mainly cuz its got ~2.5liters feeding each turbo. Now, if i had say... a 350, i could get away w/ a pair of 50trim T4's...

BUT....

the problem w/ STraight T4's and T-series turbos, is they dont have internal gates... which means you have to make a provision to mount an External gate... and spend ~$250 or more for a middle-of-the-road wastegate, and also rig up a way to mount it. Ive seen some exhaust housings that have a flange to bolt it right onto the side, but most of the time, i see WG coming off the manifold/header collector. This complicates construction and is not conducive to producing a mass-marketable turbo... but then again, people who are willing to run a turbo like this already know its a given that its gonna require a new/custom manifold and external WG and all the good stuff... so we already know its a larger commitment to run a really big, externally gated T04E-60trim than it is to run a moderate, internally gated T3/T04E-50trim.

But, you can get the same turbo in either series:

T3/T4-60trim
T4-60trim

they will both produce the same amount of boost, but one is cheaper and will be easier to find the right size exhaust housing and turbine wheel to install on a B16B or SR20DE or Ecotec :o
but asking "which is better, a T3/T4 or a straight T4?" may be like asking "which is better, a Ford V8 or a Chevy V8?" they both have specific applications that they would be ideal for, but neither one would substantially outshine the other... you know? kind of a loaded question i guess.