The TC18 has been in production for several years, not sure, but I'd guess about 4 or 5? The TZ18 is a TOTALLY DIFFERENT type of tractor and a DIFFERENT FRAME SIZE than the TC series.
The TZ (18 and 24) series is a SUB-Compact tractor series. It is very similar to the Massey GC2300 or the Kubota BX sub-compacts.
You cannot compare the TC to a TZ in any logical manner because they are designed for different purposes.