I think SCCA is much better organized and run in the short time I've run with both clubs. ER tends to run a lot at Pennington which turns into 1 car on course and a very long day. I also don't like the fact that you have to pay extra to get 2 more Rapide Pax runs when street tire class people are pretty much screwed with no index to compensate.
That being said I will run very few ER events this season and probably a full SCCA schedule. I think Solo 1 is road racing/runoffs and Solo 2 is autocross, someone correct me if I'm wrong. I think a rollbar is acceptable provided you use the correct restraints, I don't know if it will change your classing. If you're mostly autocrossing with little track time I would just skip the bar. The chances of rolling a Miata are slim to none at autocross.