Reminds me of when I was starting out. I wanted to make my own mix out of worm castings and peat moss. I accidently bought the long stranded peat moss for hanging baskets. I think I still have that bag in the garage somewhere. Any who, back to your question...
If you haven't opened it you could probably exchange it. However, if you did open it and can't return it then wet it, grab some between your fingers, and break it apart in your hand. You'll looking to see if it's to sandy and wont' hold water or if it's to heavy and clumpy like a wet sponge. If it's sandy or just drains to fast, add some coconut coir or a seed starting mix. If it's clumpy then add perlite or vermiculite.
One shouldn't really be any hotter then the other. However, regular potting mix and garden soil are both designed for established plants, not for seed. For germination you really want a sterilized medium that's very fluffy or airy.