Josh returns early December. I think Anton might have another job lined up in late November so he could leave mid November. Not sure though. I'd say if you want to see Anton try and go before November (he does have a few holidays booked - check his Facebook page for those). For Josh early December till late February is the time to go - he might stay longer but that hasn't been decided yet.
And usually the understudies do Tuesday and one of the Sunday shows so try and avoid those days. :-)
And I think they are both really good so whatever you decide I'm sure you won't be disappointed. I saw Anton in Hamburg during his first run and really liked him. Personally I prefer Josh because a) he really has the most stunning voice and b) he is so incredibly detailed in his movements. However, Anton is the original German Tarzan (Swedish/German ;-) ) and I am really looking forward to seeing him again at some point this autumn.