IndyJim wrote in another thread:
[quote] Steve this is probably a new thread but I for one would be interested in a write up of the SM vs. SE30.
[/quote]
Jim -
Not sure what info you are seeking, but let me ramble a little bit.
A little background: I did a track day in my street car at Road Atlanta in October 2004. My instructor said, “Well, you’ve just had your first hit of crack cocaine!!” How prophetic!
Did a couple more track days in November & December. I started thinking that it was stupid to be putting my daily driver at risk on the track.
A friend said “we ought to buy a Spec Miata together and go racing!” Three months later I had gone through Panoz to get my SCCA license and bought an already-built SM. I had read about SM in a lot of car magazines and grew up hearing about SCCA racing. He still hasn’t bought a car or tried to get a race license. If we could get a Car and Driver writer to drive a Spec E30 and do an article on NASA racing, maybe SE30 would become a gateway drug alternative to SM.
As I began racing SM, I met and became friends with the NASA Southeast SM director, Jason Holland. I then got a NASA license and started racing with both sanctioning bodies. Having virtually identical rules really helps produce large fields in both. Advantage: Spec Miata.
After about 6 months of track days and races, I realized that if I got my wife interested in driving I could do twice as many track weekends (“Honey, let’s leave the kids with my Mom and we can spend some quality time together at Barber…”)) She fell for it and now is the terror of Peachstate PCA Group 3. 
As she started doing more events, I began worrying about having her in an open-top car. I convinced myself to build a new Spec Miata with seat sliders so we could both drive it. I also wanted to build a car that I knew everything about. Super safe cage & bulletproof reliability were my goals. During the cage building it became obvious that a sliding seat would not give me the head room I need (I’m 6’2" and she is 5’4" so a slider is a must).
I had been watching the SE30 races at various NASA events. Ever since terrorizing Athens in my college roommate’s '84 318, I have loved the E30 body style. Combined with the sweet sound of a straight 6 (reminds me of my high school buddy Joe’s 280Z) it is a much more appealing machine than the Miata to me on a visceral level. But I do think the Miata can be credited with re-igniting the sports car craze.
I think most SE30 guys are BMW fans (fanatics?) who get into racing. I think most SM guys want the most competitive entry-level racing they can find. They don’t really dig Miatas, per se, but they are cheap to run and have big fields.
Partially as an experiment to help my friend, Tom Fowler (OPM Autosports), expand his trackside service and race car rental business, I encouraged him to build an E30 for me. His business is about 90% Miatas now and I see SE30 as a way to broaden his base.
The other motivator was to build a car I knew I could fit a slider seat in. All in all, the E30 is probably 10-20% more expensive to build than a Spec Miata. You can get on track for under $10k in either SM or SE30 if you do a lot of work yourself and put in the bare minimum for safety equipment and new parts. A tub-up build is, of course, a lot more. But similar cost either way.
OK. So that’s how I got here. After one whole weekend of racing an E30 at nationals, let me share my opinions.
The car is much less of a handful than I thought it would be (in spite of the size/weight disadvantage vs SM). The motor pulls strong and sounds great.
The Miata’s suspension is much tighter and more adjustable, but the E30 can be properly corner weighted with spring pads, ballast location, and if you are creative, non-ballast location (a cool suit box, an extra fire bottle here and there, a spare tire filled with air that measures 100% humidity, etc. 
)
I feel a bit safer in the E30 (larger cabin, kick-a$$ 1.75" cage tubing, bigger windows to exit). But I feel like I can throw the Miata around easier to avoid trouble.
The SE30 championship race (18 cars?) had MUCH more contact than the +/-60 car “spec piñata” championship race. But the qualifying races were cleaner in the E30 than SM where there was one significant first-lap crash.
The SE30 guys seem to have about the same level of camaraderie as the SM guys. Post-race cookouts were happening for both groups.
SM is farther along the curve of people exploiting grey areas in the rules, so more loopholes have been closed. SE30 would be well served to take some pages from them. Want to rule out $40/gallon race gas? It’s easier now than later. Maximum HP & torque rule? Sounds like a good idea as a backstop for the specific engine modification rules. But right now I don’t think anyone at nationals had a to-the-limit “pro” motor in SE30. I think 80% of the Miata field did.
SM has been dogged by parity concerns between the different body style & engine combinations in the 1990 to 2005 models eligible for the class. The upside is there are plenty of donors. Downside is you get some “car of the year” builders who wait to see the rule changes, then build a new car each winter. SE30 is effectively a 325is-only class, but it would eliminate the C.O.T.Y. possibility if you sunset the 318’s, convertibles, etc.
SM has a significant advantage in the manufacturer support they get from Mazda. Mazdaspeed sells parts at a huge discount to racers. BMW parts are tougher to find and more expensive.
My plans are to keep racing both cars. I don’t know how much I will do it on the same weekend. I don’t think it helped me at nationals to switch back and forth. Fifteen minute sessions didn’t give me enough time to adjust my marks, driving lines, etc.
If the SE30 crowd could get traction with SCCA to have identical rules that would allow the cars to be competitive, that would provide a huge shot in the arm for the class. I don’t know the politics, but I know there are too damn many classes in both sanctioning bodies. If you have a class that can’t field 5 cars for the national championship, kill it. Sorry. Rant over.
Anyway, thank you guys for the warm welcome and I look forward to meeting more of you at a track soon!
Steve D.