Aside from the extra 4 amps of power that the PH180 offers, it also provides fantastic circuit protection with fold-back power regulation. (edit: I must have been thinking of something else. As GGG pointed out there is not fold-back on the PH180. The breakers are described as being very good, however. Going to have to dig around to see what I was thinking of with the fold-back.) I (still) believe these are the best power sources money can buy as far as a constant voltage source goes. I am unaware of any problems or negatives about these, or have not seen any mention of problems that I remember. I'm sure there have been issues and expect other to comment on such problems.
The z1000, on the other hand is a simple transformer with a 6 amp circuit breaker. it is just fine for what it is, but is not any better than using a post-war transformer with a fast-acting circuit breaker. The z1000 brick also has one minor design flaw, in that a connection between the 18VAC hot and the 14VAC accessory hot makes an unfused short through the transformer. It is unlikely to be a problem for most folks, but it is a problem if your accessory wiring gets crossed with the track wiring somewhere.
If cost is a consideration, you're best bet may be to use post-war KW's or ZW's in poor cosmetic shape, add a quality circuit breaker, and save some dough. If cost isn't an issue the PH180's are awesome, and for the very high dollar, the new ZW-L offers all the fancy bells and whistles... and is actually a pretty fair price if you need all the power it delivers.
JGL