I have owned both of the MXRs.
ZW I would say that its a pretty good boost... but is simply shite. If you have the buudget to buy a spare one, I would recomend it. (just to clarify, Woogie just questioned me) There was a faulty batch of them apparently, his is fine :P but to me it didn't feel as rugged and was just NOT reliable, The pots also had issues as well as the switch, which was apparently the fault with the batch
GT OD - more powerful. MXR said to me:
Hi Will,
The GT-OD is a delivers a true overdrive with more low end than the ZW44 and
is overall a quieter pedal. If you are looking at overdrives I would also
suggest the MC402 MXR CAE Overdrive Boost. It is smoking! I hope this
helps. Let us know if you have any further questions.
and
Hi Will,
Reliability will not be a problem. The GT-OD has a different switch. The
new ZW-44 switch has also been replaced with a more reliable switch. We are
aware that there was a faulty batch that went out and that problem has since
been resolved. Please let me know if you have any further questions.
Sorry I am useless with the quotes.
I do not suggest emailing them, unless you want your purchase to be in 2 months :P
The GT OD actually has a bigger switch unit as well, also the casing is slightly bigger, the knobs slightly larger and feels more rugged. I enjoy both a dynamic type setting and creamy overdrive setting, but most often settle in the middle. Although I dislike the tone knob - as i do on most pedals however
Edited to make that more clear about the reliability