I had the same concerns as you with regards to jumping straight on a big bike, but went the DAS route anyway. I struggled getting up to speed on the 125. The 500cc doesn't take long at all to come to terms with, felt comfortable in one lesson.
Also, one of my mates went the 125 route, failed twice, but his instructor kept saying to try the big bike because he would finder it easier, so he did, and it was a case of 3rd time lucky.
IMO DAS is the best approach, but then I know no different, and only ever rode 125 in CBT, I also passed before the new swerve test came in.
But there are some that'll say you need to learn on smaller bikes and work your way up to learn how to ride properly.
Edited: 09/06/2009 at 19:01