Currently there is no way to have only one of the packages installed. I'm using mdraid based software raid and Boot-Repair requires installation of both packages.
There could be some stability problems if mixing mdraid with dmraid. Dmraid is for fake-raids used via MBO chipset. In some rare cases there could be some problems if disks/partitions contain metadata marked by one of the drivers and the other one misinterprets it. Didn't happen to me but there are some bug reports on launchpad. Dmraid does not come installed by default in natty due to some problems in lucid if I remember. Don't remember if installing *raid pkgs triggers initramfs update, but if it does and the machine won't boot anymore it could be quite a challenge for a newbie to fix this. PErhaps Boot-Repair could detect which raid is used and only install apropriate package?
Thanks Alen for the bug report.
In a first time, I think we can just let the user choose dmraid or mdraid. This should be easy to implement and would solve this bug.
Then, the problem is that some users may not know which one to choose. So we need to find a reliable way to detect which raid is used, in order to propose only the appropriate one. I don't know how to do this at the moment, and we should create a new bug report (wishlist) for this.