If its to improve posture, then Alexander Technique and pilates should both help. Personally I've found AT to be of huge benefit to me, much more so than pilates. I continue with the pilates because I've been told by so many people its great for low back and pelvis problems, even though I don't think its made any difference to my pain levels.
But everyone is different, and I've heard of some people with our problem who think that pilates has really helped them, so it could be worth trying. One positive effect for me is that I now have the flattest stomach I've ever had in my life, so it will improve how your body looks, if nothing else!!
What I like about AT is that it deals with the whole body, not just your 'problem' area. I think this is important because you soon realise how much everything is connected - how your head position affects the position of your pelvis etc. I can't recommend it highly enough. It would definitely help with your degenerated discs because you would learn to stand, sit and move in such a way that it puts less pressure on your discs. It won't cure the problem, but it should help prevent further deterioration.
I'm not sure if Bowen would help with posture - I think its more like a massage type of treatment, concerned with releasing tight areas of muscle. I'm sure thats a very simplistic explanation and no doubt someone with more knowledge than me can help with this one - the only experience I've had is when I got someone to do Bowen therapy on my horse!