The only thing I did not see listed above to consider, and I may have missed it, is that mummy bags tend to come with hoods and rectangular ones generally do not. If you are picking one without, you may need a hat or "watch cap."
A couple people mentioned size. Make sure you fit in the bag. After that you can consider other comfort factors, whether you want to be able to move much in the bag or whatever.
The choice also depends on budget, how much the money will mean to you, considering how much you plan on using it. Further, in the climate I am in I do not generally need that much of a bag and I do not get out that much, so I can make do.
You can have one made.
For example:
http://www.warmlite.com/start.htm
(If
nudity bothers you, avoid this site.) This warning should make sure everyone checks out the link, either to enjoy or complain!
Some of the design techniques and ideas are interesting.
Another site with some different ideas:
http://www.wiggys.com/
Recently saw some favorable comments on the Outdoor Forums at
http://www.jungletraining.com about recent use in the jungle of the SnugPak Jungle Bag.
http://www.snugpak.com/codegreen/sleepingbags.php4
The thread was here, but you have to be a "member" to access it. (free.)
http://www.jungletraining.com/forums/sh ... stuff+sack