Magic systems also differ from game system to game system. Again, we prefer Pathfinder. We like the time it takes to cast the spells, the possible concentration checks, and the power levels. If you use my world, there will be some mixing and matching of spells, (such as for the Clan Warriors who have both mage and clerical spells), but that can be done with any system. Don't get wild and crazy, making the class more powerful by giving out all the powerful spells for both classes to one class, but adjust as you see fit. It's your campaign and you should have a say in how it runs. Book rules are great starting grounds for everyone, but after some time behind the GM screen, (or years in my husband and I's case), you get a feel of what will work and what won't!