Hello Pete I find myself in a similar situation but instead I'm a 3ds user. Recently two friends of mine used one of those two services you have mentioned-I'm not going to say a name as I don't know if it is appropriate- and they were very disappointed. A large number of frames came back blacked out and there was no support, to guide them through in order to solve this issue. Anyway I believe that there are many, smaller size, render farms out there with lots of experience and more friendly customer approach. I'm still looking for one though! Will let you know if I find anything