Mr. Losi, the A & B does not change the stiffness of the insert, it only designates the thickness of the insert. The 'A' inserts are thicker than the 'B' insert.
Mike F, I would go with Sorex 36R's. The 32R's would work well at the lower end of the 100's but has a narrow effective temp range. I have run the 36R's in temps from 85 to 170. All you need to do is vary the insert and possible take a practice lap to get them up to temp (in the lower temps). For 100+ temps I would recommend you go with a 36R with Sorex Firm or HPI Blue inserts. I also like to cut the circumference of the insert down (approx 9-10mm) so that it fits tight on the wheel and creates some air gap. It take slightly longer to bring the tire up to temp (1-2 laps) but in my testing it was .2 sec. average per lap faster than a full insert.
Hope that helps.
TLR, Spektrum, Tekin, SMC, MO-GRIP, www.factory-rc.com https://www.shapeways.com/shops/MPP