Just to add one further comment to what is a good reply from Jess, if on the new pension scheme, your pension will be final salary based, as opposed to rank dependant, so that for example, someone on the PA spine may well end up with a substantially larger pension under the new system, compared to the current system, but only if you stay to age 55
To elaborate a little on what Jess said, part of the thinking is that you can retire at your 18 year point, with your EDP, and still have enough working years left to start another career and earn a full pension from that career as well.
The MOD website explains in more detail about the new pension scheme, and it has also been discussed in earlier threads.
Bottom line is ..... if you join after 01 Apr 2005, you will be on the new pension scheme. Suggest you try and join before then, at least you would then have the option of old or new.