Well the RB20DET (R32) is an all round good performer and has rather strong internals, so can handle like up to around 240rwkw with the standard internals.
With the 180SX's, they came with the CA18 and the SR20. In my opinion the SR is the better of the two engines and response better to mods, even over the RB20DET in some instances. Both the 180 and the R32 handle excellent, but if had to choose the 180 would be slightly better with handling. But what won me over when i bought my R32 was the appearance. It looks so much more stylish than the 180.
As for parts, there are lots for both...so no problems there.
For further info about the R32 GTS-t, do a search on here or visit www.meggala.com
Best thing to do if i were you would be to test drive a couple of each to see which you prefer.
Regards,
Scott.