The T-4 Medium-Hard compound tires (Durometer 60) offer good grip plus great wear characteristics and even decent wet performace (but not in standing water - they are slick), but they are a little more squat in profile and offer slightly less available lean angles than the 6" setup.
The T-41 Medium-Soft compound tires (Durometer 55) are a medium compound and offer a bit more grip but at the expense of increased wear characteristics.
The R compound tired (Durometer 40). Soft, are primarily for rain use & are available as slicks for hand grooving or with a hand-cut rain tread pattern. These tires offer the best possible rain performance but are extremely soft and will get hot, oily, slippery and go away very quickly if used in dry conditions.
The B compound slicks (Durometer 65) are a hard compound tire & are generally not used in competition. These are favoured for rental applications for their long wear characteristics.