I have three types of bivvy bag:
I have a super expensive hooped bag which i haven't used in years (it was very useful on the moors with no tarp though). If you are going to be using a tarp in conjunction with the bag, then i wouldn't bother going for a hooped version.
I have the issue bivvy bag (old type, and the new type with the wider neck opening). These get by far the most use as they are roomy, and super strong.
I have an Alpkit Hunka (standard size)- this is very lightweight, thin, and rather on the small side. That said, i bought it for its lightness (less than half the weight of the issue bivi bag), and it is just about long enough. It's breathability has been fine on the few occasions Ive used it, but in theory it isn't supposed to be as breathable as a microporus type membrane (like goretex).
I have never had any problems with not having a side zip (or indeed, any zip) in my bivy bags- for me the 10 seconds or so longer it takes you to get into the bag at the start of the night is more then outweighed by the potential area for the bag to leak that can be presented by a zip. The later issue bivi bags are dead easy to get into without any zip (even in a hammock) as the head opening is so large, you can pretty much open it down to your waist.
I've just noticed in the above post Ive used three different spellings for Bivvy (or was that Bivi, or Bivy?!).
