Here's some points of comparision from my own experience (limited as far as Z1 is concerned). You could also try asking VC at the VideoLifestyle forums. He has both of these cams.
The Z1 has separate motors for zoom and focus, the XH-A1 has only one. This causes a problem when you are in autofocus mode and then try to zoom. The focus tends to drift in and out. (Solution is easy - just don't use auto-focus - but it is an XH-A1 limitation nonetheless).
The Z1 has shot transition which is wonderful. You can set up 2 shots, with different focus point, zoom level and even exposure settings, and then transition between them over a selectable time interval. The XH-A1 can transition between 2 focus points or 2 zoom levels, but not both at the same time.
Battery life is similar for the 2 cams.
The EVF eyecup on the XH-A1 is useless, and you cannot see anything in bright sunlight if, like me, you wear specs. The Z1 has a big rubber cup that works great. What saves the XH-A1 though is that you can buy a Z1 eyecup and it fits nicely over the top of the XH-A1 cup. It's simple but brilliant!
The Z1 has a much better and bigger LCD panel than the A1. But since my previous cam was an XL2 which only has an EVF, the XH-A1 panel was a big stepup for me and I find it is OK. Useless in bright sunlight though, unless you buy a hood.
The XH-A1 has many parameters that you can adjust to change the appearance of the image. This allows you to get a certain feel or look. It also helps to compensate for particular or difficult lighting conditions. Bit of a learning curve but definitely worth it. There are also many user presets that you can download for free if you don't want to make your own. I don't know much about the corresponding Z1 features, sorry.
The XH-A1 has a nice big aperture ring that I use all the time to control exposure. 99% of the time I use 1/50s shutter speed, and adjust the aperture on the fly, as necessary, while shooting. Of course there are AUTO (auto aperture and shutter), TV (auto aperture), AV (auto shutter) and full Manual modes as well. Z1 has aperture adjustment but I think it uses a small wheel at the front of the cam. I believe the XH-A1 adjustment should be finer but don't know that for a fact.
XH-A1 has much better implementation of progressive mode than Z1 does. Although they both use interlaced CCDs, the XH-A1 25F does a better job of producing the 25p cadence, and with little loss of vertical resolution.
I considered the Z1 (for a long time) and then ended up buying the XH-A1. Reason? It has a sharper picture and better colours than the Z1. Bottom line is that both cameras are good, and I would be happy to use a Z1 if I didn't have the XH-A1. Would prefer to have the lens of the EX1 even more though......
