A Miata 12V battery is on of the choices you have for the 2001-2003 Prius.
Other choices are:
1. Prius OE battery
2. Optima battery
http://www.elearnaid.com/opbaandkitfo.html
3. Odyssey battery PC925LMJT
http://www.odysseybattery.com/batterysearch.asp
If you have a 2004 or later Prius you have these choices:
1. Prius OE battery
2. Optima battery
http://www.elearnaid.com/12vo1topraub.html
3. Odyssey battery PC925MJT
http://www.odysseybattery.com/batterysearch.asp
All these batteries are AGM (advanced glass mat). This means there is no liquid in them so if you are in an accident they won't splash acid all over.
What's different about them is:
1. The Prius OE battery will survive one, maybe two total discharges. No kit is necessary for installation.
2. The Miata battery will survive one, maybe two total discharges. The Miata battery isn't quite large enough for the 2004 and later. You will have to fabricate an installation kit because the terminal are different. It has been sold at a lower cost than any of the others.
3. The Optima battery will survive many total discharges. If you buy from the link provided, there is a kit included.
4. The Odyssey battery will also survive many total discharges. It's reputed to be better quality than the Optima. You will have to fabricate an installation kit because the terminals are different and the battery is a different size.