kt, yes, it's possible that you would need to pick and choose a supplier based on what your priorities are, and what their 'speciality' is. A traditional S&S-only provider probably wouldn't offer a great interest rate, but would have a better range of stocks to choose from, while a company that hasn't offered S&S ISAs before (I can't think of one off the top of my head but I guess there might be some) would be the opposite.
I would imagine that the main players (Post office, high street banks & B Socs etc) will offer both cash and S&S on or very soon after the regulations change, but the smaller players might decide to stick to one or the other. However, the smaller players can sometimes offer better deals if you are certain you only want to stick to cash or S&S.
Another issue that needs to be worked out (by the Gov't and the providers) is Fixed Rate Cash ISAs, and what will happen to them. My cash is in a 2 year fixed rate, which doesn't end until April 2015, so I won't be transferring out of that, but I won't be putting any further cash into it either.
Wow, who knew I would have so much to say about ISAs! :-)