Should I install Ubuntu as UEFI or Legacy?

I began to install Ubuntu, but got a message about UEFI/BIOS compatibility and how I wouldn't be able to boot both.

If I install as UEFI, will I be able to reinstall Windows 7 as UEFI? Or should I just install Ubuntu in legacy mode?

To prevent needless complication, I would advise that you install Ubuntu in the same fashion that Windows is currently installed.

So, if Windows is installed as UEFI, it would be best to install Ubuntu under UEFI such that you don't need to swap bootloader modes or reinstall Windows to change OSes. Similarly, if Windows is in Legacy mode, install Ubuntu in the same.

If it really doesn't matter to you, though, I'd highly suggest using UEFI. It offers many more features that can come in handy on modern systems, and can allow for better compatibility with graphics and hardware stacks on modern systems. All in all, though, it really doesn't matter after you get past the bootloader.

if you choose to install both in UEFI mode make sure you have an EFI partition (250mb will be enough) as without this partition your OS will not boot

(if your windows 7 is in UEFI then i'm pretty sure you have this partition already)

Does Ubuntu need this EFI partition?