Also, if you use Logic, it's not a bad idea to get the emagic products to match. Emagic removes some midi latency when you pair the unitor/amt with logic. nearly anything will work, and I've had no bad luck with any midi interface. My Amt8 is solid, and has no more or less functions than most other midi routers. I imagine the amt4 is just as good. I chose this one because it had both usb & rs232/serial, so I could use my old laptop with rs232, or newer machines with the usb. It hasn't skipped a beat.
Really the biggest thing I'd say to look for is when the machine is connected to the PC. Most of them completely surrender the router functionality, and put the PC in control...something to be aware of. So you'd need to have the PC perform router functions when in that situation.