If your group will cover you, you should have the same coverage options as anybody else covered by the same group. Your diabetes as a previous condition shouldn't make any diffrence.
The HMO is going to be managed much more closely and, as Zoe says, will generally cost you less out of pocket for a wide variety of services than a PPO.
When I first started working for my current employer, they offered a BCBS HMO/PPO plan which really was the best of both worlds. None of my diabetes specialists team was covered by the HMO or would participate in the BCBS HMO program, so the PPO came in handy even though it was more expensive to see my specialists.
Eventually, either my employer or BCBS, I'm still not sure which, discontinued that service and I had to choose. Going with the PPO was an easy decision for me because I had been seeing most of my specialists since my diagnosis. I didn't see any reason to go with the HMO and their care providers at this stage in the game.
If you are starting from scratch with your specialists, my GF has Kaiser HMO and doesn't have any complaints about the level of care their specialists provide. Granted, she does not have diabetes, but she seems happy with Kaiser.
Moving over to a CGMS and a pump was pretty painless through my PPO and any delays I experienced was not because of my insurance coveage. It did take awhile but BCBS PPO, most definitely, was not the bottleneck. Like Zoe, there was never an issue regarding whether or not the PPO was going to cover them as DME or which pump I wanted to go with and they cleared it pretty quickly.
I actually pay 30% for my DME which played a bit into my decision to go with the Omnipod because it has a significantly less up front cost. I will pay more over the life of the pump, but the higher maintenence cost at 30% is not that bad.