Consider using the new NetVanta 3140. That unit has 3 10/100/1000 Ethernet port and is specifically designed to support 2 ISP connections like you describe. It's good for up to 150Mbps of throughput and can do the VLAN tagging, etc that you're already doing. Basically it can do everything the 5305 can do minus the physical reality of not having DS3 or T1 cards, slots, etc and of course no option for a redundant power supply like the 5305 had.
Adtran Applications Engineering