What bike is more suitable for my situation, 250cc sportbike or 500-600cc cruiser/standard?

There are several smaller bikes that are great to learn on that will be just fine for your commutes. In fact, there are too many for me to list here. I'm a member of a forum and a website that is great for beginners. Check it out. It's got reviews of alot of different motorcycles and I'm sure you'll find one that will fit your needs. Feel free to join and ask questions in the forums, too. No matter what, stay safe and wear your gear.