To answer the first questions--yes, it is worth it to have your own tanks. I have never regretted buying my tanks. Since I bought my own tanks, I dive more often than before I had my own tanks. There are some instances where having your own tanks is much better than renting:
If you're interested in going after bugs during mini season in Florida, having your own tanks is a must. Rentals are often sold out for mini season.
I get my tank filled right after diving and have them ready to go for whenever I have a chance to go diving. It is especially convenient when I do a mid-week night dive. I don't have to use my lunch hour the next day to return the rental tanks.
Having your own tanks also allows you to get the best tanks for you, whether it be smaller tanks if you're a smaller person, or larger tanks if you need more air.
Also, you can get tanks with the buoyancy characteristics that fit your needs.
Bottom line--if you have your own tanks, you will dive more.
Ron