What is the ideal for Quebes, legacy bios or uefi?
They are different. I think qubes development team recommends BIOS, but there are cases of hardware that only supports UEFI.
In the (hopefuly) near future UEFI may become attractive, with the implementation of safeboot. That will increase the security of the OS significantly.
A better yet solution (only for a very narrow range of hardware) is heads
Probably I would choose BIOS if I had that option, because it is less complex, but this is very likely to change in the future.