Basically the T34 is a bigger laggier turbo which comes on boost later which is capable of being tuned upto 350 bhp
The T25 is a smaller less laggier turbo which comes on boost sooner but is only tunable upto about 280bhp

In standard/stage1 tuned state the T25 is better as the boost there earlier. the big turbo is better for more power thou

They are phyically different in size and the T25 has a triangular looking take off on the front housing for a recirc (no noise dumo valve)