[member=4271]Chrishogguk[/member] It's definitely worth trying both - for a free trial and then for a paid month, so you get access to the full range of software and you can see what fits you the best.
I've used both. Obviously PA has a much bigger forum presence, but in terms of finding someone to help you if you're having a problem then there's no difference across the sites for me - tag a mod and you get an answer. You see some prominent members on both sites who seem to add a lot to their respective forum and offer good help/tips/advice. Forum presence is a strange one though, isn't it, because really what does it even mean? That there are more people there to comment "made £20 from this", "Refund for me", "I used the Top Cat slot", "Good match on the away team in x game with Smarkets" and asking how to do offers - it's all about how much useful information you think you can gauge from having more members on a site. It is nice to see whether a slot offer is worth doing judging on how much people have won/lost on average but then again I think that people who make money are more likely to post so that might be a bit skewed. I feel like I've been doing MB long enough to have a good sense of if an offer is right for me/worth it and what slot to use for what kind of offer. If you need guidance on that sort of stuff then PA has the edge. OM is catching up, though. PA's mug thread can be useful but then I think "Oh and suddenly 2000 people put the exact same bet on within 30 minutes so maybe I'll avoid it actually." I am a fan of the 'Make £x per day" thread on PA.
I used to have an issue with the PA oddsmatcher and found that the OM one performs better, however nowadays I don't use an oddsmatcher at all, I'd rather find my own for football and then if I'm doing any horses then I use the racing matcher or each-way matcher. The price per month is better for OM (and you get a lot more software which performs well) but there's no acca software (personally I wouldn't use it anyway as it seems like gub city unless you're very careful with keeping your accounts in order) but then the price per year is the same. I've read some say that the PA Match Catcher is a better piece of software, and from what I remember of the free trial of it that I was given, yeah it is pretty good and worth it if horses is your thing however- that being said, if I'm paying £10 a month for a piece of software on top of the normal subscription fee, which is more than OM to begin with, then damn right it had better be better than OM's equivalent (which is inclusive in the monthly subscription fee). You hopefully get what you pay for! If you do a decent volume of horses then it's probably worth paying extra for, but the refund offers aren't my bag.
I personally think that now there is more competition in the MB services market (including the free sites) it's causing companies such as PA and OM to up their game. I wouldn't be surprised if by this time next year there is a new major contender vying for the PA/OM customer base. I think it's pretty good that a bit of healthy competition is pushing the other to be a better site. A lovely aspect of both sites is the 'no strings' ability to jump back and forth. You do PA for 6 months and become sick of it? Cancel it and jump ship. Decide OM is not for you and actually PA is better suited to how you MB? Cancel it and pick up where you left off with PA. It's nice to be able to do that!
Sorry, I ended up writing a lot more than I intended!
TLDR; Both have their advantages and disadvantages. It comes down to personal preferences. It's easy to try both, so try both! You'll find what fits you best ;D