Jacquelynn, you are wise to ask around before jumping in to ANY online proofing solution... because ideally, you don't want to be changing a client-facing interface too often.
We converted our online proofing from Pictage to PickPic last year, and couldn't be happier!!! Along the way, I did evaluate Millers -- primarily because we use Millers for much of our lab work. They are a FABULOUS lab, and I've recommended them to many a fellow photographer. However, I would proceed cautiously with respect to their online hosting...
I'm not sure if it's still set up this way, but last year they actually had two separate hosting sites -- one for portraits/weddings, and another for sports/events. That was a bit awkward for me, 'cause I just wanted to direct clients to ONE hosting site from my website and blog.
Also, the "uniqueness" of events in their database is determined by an event key or passcode. So this requires EACH event or proofing gallery to have its own unique passcode. And while that may not seem limiting initially, we found it to be an awkward requirement because we often create multiple events with the same passcode -- primarily for our sports work with high schools, where it's often easy for folks to enter the same passcode for all the sporting events from the same high school (i.e., usually the team's name or mascot). We couldn't do that with Miller's online hosting.
So if either of these raises a flag in your mind, you may definitely want to check with the folks at Millers to see if things still work the same way.
Also... whichever 3rd party method you explore further, I'd definitely try a few events internally first... and see how it feels being a client BEFORE broadcasting its availability to your client base. It's a bit tricky with PickPic, because you need to "purchase" the shopping cart software up front... but then again, I haven't run across ANYONE who regrets they've gone this route.
Best of luck,
David