it's easier to have separate WP installs rather than use WPMU.
that is generally true
How many WP sites would you run before deciding to use WPMU? A normal WP install is very easy for me; it seems that MU has more issues right now.
There is a more than small learning curve in mastering managing a multi-site installation. Since you say normal WP installs are very easy, if you are less than the range of 4 to 6 installations I'd suggest you are best off sticking with separate installs. Unless of course you really want to learn to manage a multisite installation, in which case 4 to 6 is at the low end of what justifies a multi site install. But there are still a fair amount of time savings, over time, upgrading core and plugins once per upgrade rather than 4 to 6 times.
A lot of it comes down to whether you see installing multisite as an experience you want, or a pain. If you want to do it, go for it. If you don't, the single installs, at the range of 4 to 6, are quite manageable.