I, too, spent about 8 years in the mob as a mil ATCO before jumping ship to join NATS in 94 - just beating the age limit at that time. Whilst I had a great time in the RAF and have never regretted joining, I'm glad that I left when I did.
I wholly agree with the last couple of posters; civvy ATC is currently the more comfortable option and definitely the more financially rewarding. Just depends what you want from life. I think that whatever you decide to do you should do it because its what you WANT to do NOW. You'll need that driven desire to get through the selection process for either job.
Good luck.
Regards
Sad