As a recent discussion was going, since you already have something to reference (the ER6i) to, you should probably think in terms of the sound signature or quality you're looking for.
In other words, do you have any gripes against the ER6i, or anything you really like about it?
If you want to try out a different sound without paying too much, the UM1 is certainly that.
If you're going fork out more money for an upgrade, you could look at the ER4, E4c or the SF5Pro (coming from and loving the ER6i, I don't even give this any consideration, but that's just me).