If you are upgrading this late in the game, spend an extra $10 for 8x4 channel bonding.
6121 is 4x4
My recommendations are as follows:
1) Arris cm820 (Be careful here, make sure you get it from a good vendor)
2) Cisco 3008 - Probably easiest to get for a good price
3) Moto SB6141 - Upgraded 6121 with 8x4 channel bonding