I doubt I am the first one to request this, but please consider it a +1:
Please let the user decide the installation location of each engine version.
The basic reason(s) for this:
You installed the launcher on one drive and it gradually filled up over time. You want to add a new engine version, but there is no space on your old hard-drive. For compatibility to older projects and older example projects you need to keep the old engine version. So you have to move everything else on the same harddrive or you need to hack around in the explorer (which might not work properly), or you need to uninstall everything and reinstall it on the new hard-drive.
You also might want to have your main development engine on an SSD (which fills up quickly…) and the other engine versions on a slower hard-drive.