I really do appreciate the advice everyone! It's alot of bucks to spend, so I'd be wanting to make an educated decision! Here's the low down put simply:
1. A 12" scope would cost less so I would be able to buy it sooner
2. If I had a 16" it would stay set up the majority of the time in my garage and be moved about 5m from where it sits; I have a flat trolley that I currently use for my 8"; I'd say that with a bit of "jimmy-ing" I could move a bigger scope with it
3. When I look through an 8" at Jupter, I see a little detail (mind you, I've only ever looked when its low to the horizon and Spring is not necessarily a great "seeing" season); so how much more detail is a 12" going to show me? I do like looking at DSO as well but really prefer Lunar and Planetary (but not photography at this stage!)
4. Thanks Brian for info on the "off axis mask"; did google it, but opinion seems to be divided

on the subject!
5. Getting a 12" may free me up to buy some grander eyepieces!
So there you have it! Aren't you glad you're not me!