Question: What insulation should I use with 2x10 joists on 12" centers?
Answer: As far as R-Value goes, you can use R-30 or R-30C (cathedral) depending on your preference.
Owens Corning makes a 12" insulation product. If it is not available in your area there is also a 24" product that can be cut in half.
For your information, there is a Compression Chart on the Owens Corning website that indicates the R-values of fiberglass insulation products when they are compressed into smaller spaces than they were designed for. There is also a Frequently Asked Question on the topic of compression.
Bottom line: A little compression is sometimes unavoidable but you can maximize your energy saving opportunity by using the product with the highest R-value that is the closest in size to the space you have to insulate.
