Modems aren't s***. If you went through 5 I would say the s*** was the bull s*** those techs were throwing yourway by replacing it. The vast majority of connectivity issues is not hardware, but signal levels going into the modem.
Generally though, I would suggest getting separate phone modem and data modem for a number of reasons. First, it makes troubleshooting easier. Second, it allows you to put the MTA closer to the punch block(IE basement). Third, I find that the firmware of those units are sketchy. They are always patching them. And last, you get to keep a data modem, you have to return a hybrid. Just compare a SB5101 with a SB5220.
As for the worth of a DOCSIS 3 modem outside a Ultimate package, it depends on your area. Try calling your ISP and seeing how many customers are on your node. Or look at the forced idle on your QoS handle. If both of those are good. (Under 400 usually and under 50%) then a upgrade may be worth it. If their squashing their traffic or if your area is under strain, your probably not going to see much of a improvement.
Last if your going to DOCSIS 3, I highly suggest making sure your signal levels are perfect. RX 0-2, TX 46-48, DS SNR 36+. I found the modem naturally gave RX levels 2-3 above previous. Just FYI