Personally, I've had a bad experience with hydor minis--I only recommend adjustable heaters. Since the minis only heat the water a few degrees above room temperature, if your house gets into the low 70s in the winter, your betta will still be cold. If your betta ever gets sick for whatever reason, you want to be able to have stability and control so that you can react appropriately to the situation. Besides, adjustable heaters are flat out higher quality products--my last mini ended up cracking right down the middle of the plastic.
I use and recommend this heater: http://www.fosterandsmithaquatics.com/product/prod_display.cfm?c=3578+3743+11368&pcatid=11368
If you can, you should try to do all of your shopping online. Pet stores have a lot of overhead because of how they work, so they're forced to jack up prices in order to still make a profit. Buying just a few items at a time online will more than make up for their very reasonable flat rate shipping.
If you want a cheap Quarantine Tank/Hospital Tank solution, look no further than the rubbermaid/sterilite plastic storage bins that you can find at wal-mart or target. There are some good clear plastic options that are safe for fish, safe to heat, and ideal for hospital situations because they're long and shallow--optimizing swimming space while making the surface easy to reach for weak fish. 4-5 gallon clear plastic storage bins are only about $3 a piece. You can't beat that.