Whatever one you chose.. You don't install Desire ROM on top of FRF91.. nor do you install it on top of CM 6... It's a totally different ROM, that uses FRF91 as base, just as CM 6 is a totally different ROM which uses FRF91 as base.
Evil D: on xda has a much more mature Froyo Desire ROM port at the moment, and in fact Paul@modaco may relinquish development to Evil D and focus on other areas, while providing kitchens for devs such as Evil.
Evil's ROM has working FM radio, 99% working USB connection ( no need for an extra app or widget to mount USB on your computer! Just sometimes the icon disappears in the notification area), GOOD battery life nowadays, etc. Bluetooth is still hit-or-miss. I don't use bluetooth so not an issue for me... Desire ports are really maturing, so it's worth trying!