I'd reckon you'll get a fair go at either club Placebo. Phoenix 5s is a new team so they are certainly likely to welcome and give plenty of opportunity to new players. Civil Service 5s are relatively new at 2 or 3 seasons and I'm sure they'll give you a fair crack of the whip too.
I don't play for either side so I can't comment from experience but I do know lads who play from either team. From what I can garner the turnover in Civil Service seems to be really, really high on their lower two teams. Make of that what you will.
That said Phoenix subs are quite a bit higher than Civil Service so if finance is a factor that might come into it. Although at Phoenix they have 3 (I think he said it was 3) coaches who will help you through the year and you get some club paraphernalia so thats what you're paying extra for. At Civil Service there is no coaching.
Best of luck with it anyway and I hope you enjoy taking it back up again. The more welcomed back into the cricketing fold the merrier!