Hi xlink_nz,
Are they all much the same ?
Well now, that's a hard one to answer. But in short, really, they're definitely not all the same. That said however, the common feature they do all share are that they are all infant seats that come with bases.
I suggest you read the bottom of this page for more info on the different types of rear-facing seats:
http://www.childrestraintsafety.com/rear-facing.html#typesThe most important thing is to get a seat that
can be correctly installed in your vehicle,
one you will use correctly every time,
one that suits your child and your needs and of course - one that suits your budget.
There are a lot of reasons why some seats can be installed in some vehicles and some others can't. Reasons for this can be sloped back seats, long seatbelt buckle stalks, seat "humps" if installing in the middle, not enough room in your car for some rear-facing seats and the list goes on...
The most important thing is to get a short list of seats that appear to meet your needs (which I see you've already done) and try each one in your car to a) see if it fits and b) see how easy it is for you to install it and adjust the straps, play around with it etc.
Another seat I'd also suggest you look at is the Graco Logico S which I've personally found is
extremely easy to install. The base for the Safe-n-Sound Untiy whilst a very nice seat, can be a little bit cumbersome. Also with numbers 1 and 2 on your list, they can be installed with or without the base - which is a nice option to have. However to my knowledge the Unity can be used only with the base - it is also dependant on every vehicle you travel in having an anchor bolt.
I'd suggest you visit a retailer such as Baby On The Move (
www.babyonthemove.c o.nz) who stock 2 out of 3 of those seats you mentioned. They also have other seats and at least this way you'll get to try them in your car before you buy.