The HVX and HPX are very similar. The HVX still has the ability to record DV to miniDV as well as all DVCPro HD resolutions/frame rates. The HPX is all solid state, so no recording to tape, but you can still record DV if you want. Because it has not tape transport, the camera is lighter and has more features. I think the best feature that the HPX has over the HVX is HD-SDI output, which is uncompressed video-very handy when doing any compositing and heavy color grading. Both have XLR connection too. In my opinion, the HPX is the way to go. No need to record miniDV anymore if you can avoid it. Just archive HD and if you ever need SD just down convert, ya know.
Also, I don't know what kind of videos you plan no making, but if you ever get a 35mm lens adapter like the Letus Ultimate, the HPX is a little more friendly with that too since those adapters always flip the image upside down. The HPX allows you to flip the image so you're not looking at an upside-down image in the LCD or viewfinder.
The only difference between solid state and tape is how you ingest. Editing is the same. I don't have a solid state cams, but I know it's really not complicated ingesting P2 cards. I believe it's faster than real-time too.
"Is recording to P2 a better idea than the SDHC card AVCHD format? How reliable are the cards compared to tape?"
Well, just for clarification, P2 is just media. You can record anything to a P2 card - DV, DVCPro HD, AVC-Intra, AVCHD, any Panasonic format. So to answer your question, when I suggested P2, I really meant recording DVCPro HD. And yes, DVCPro HD is better than AVCHD. DVCPro HD is 100Mbps while AVCHD I think is only 19Mbps. DVCProHD is an I-frame codec, and AVCHD is a Long GOP. I-frame codecs are more edit friendly and MUCH better when you start doing graphics and compositing. DVCProHD is almost as good as it gets. It's used for a lot of broadcast programs, so you can't go wrong with it. It's very stable.
P2 cards are very reliable. BBC's Planet Earth was shot with P2 media. The media is very rugged. I have never heard a complaint about P2. I've heard you can drop em, step on em, drive over them with a car, drop em in water, sand, expose them to humidity, cold temps, you name it. It's super reliable.
As for archiving, ideally you want to use LTO tape, but the machines are expensive. So I suggest burning your stuff to Blu-Ray until you can get yourself an LTO machine. You can also archive to HD tape, but I don't think you want to spend the money on a new deck