Hi, Janet, and welcome.
I think you'd be happy with either. I too have been a Honda loyalist, but I chose the Prius for several reasons:
- Like you, the cargo space
- Better rear seat leg room for my tall teenagers
- What I believe is a superior hybrid system design
- The overall uniqueness of the Prius
Potential fuel economy as determined by driving conditions may be a consideration. For urban and suburban driving, especially in warm weather and with a warm engine, the Prius can't be beat (except by the now-discontinued Honda Insight). Either will do well on the highway, though some might give an edge to the Civic.
New versus used? Both models have proven to be dependable, so if I found a used model in good shape with a good price, I would consider it. I bought mine new, partly because I wanted a new car (and its warranty), and partly because at the time a used Prius was hard to come by and nearly the price of a new one. I can't speak to the tax credit issue, but it's certainly a consideration. I bought mine in 2005 when there was no credit, only a deduction, so I haven't been following the tax credit issue closely.
By battery replacement, I assume you're referring to the hybrid battery. There have been a few, but they're rare, and many of those have been replaced under warranty. There are reports now of earlier model Prius with 200K+ miles. I think a conventional car is more likely to need a costly repair in its later years than the hybrid would need the battery replaced. I've seen prices quoted in the forums in the $2K-$3K range.
You might search or post the same question on some other forums. I can't tell you about any Civic-specific ones, but here are two for the Prius:
www.priuschat.com
www.priusonline.com
You will find Prius-only (and anti-Prius) zealots on either, but you will also find many that give you balanced and useful information.