This is the IPO knowledgebase, if you want take a look.
I previously worked for the largest shoretel dealer, and they were also one of the largest (top 20) Avaya BP's there was differing opinion about which was best by the technical staff.
First of all, I mostly care about what techs have to say because they are the ones who install, and program the systems. Everyone else, including all other shoretel, and Avaya partners employees, managers, owners, etc., are just users, so they do not know much. Users know what they like, but do not know the application of the technology which is possible.
Where I worked IPO techs were a dime a dozen, including all experienced enough to implement an IPO probably three dimes worth at least. Shoretel experienced implementors were much fewer, maybe 6, but no where near as experienced with the shoretel as the IPO guys were with the IPO.
This is simply because the IPO is implemented so incredibly more numbers wise than the shoretel that it was impossible to have as many experienced shoretel guys. Very few were experienced on both, and none at the time I left this year do I believe had more than 6 shoretel implements if they had ever installed an IPO at all. Almost no one who was highly experienced with the shoretel had even installed one IPO by 100% themselves yet. That means the IPO techs were the only ones really qualified to evaluate both, which leaves a little bias in the mix most probably.
That being said, the concensus was that the shoretel was as stable as the network. For multi-site that includes the P2P's. Servers fail too because they have moving parts, so that is the next weakest link. If having just mailboxes is sustained VM, then P2P, or VM server being down does not effect sustained VM service. The shoretel is hands down more expandable, but has less features than the IPO, which is irelavant if you don't need what shoretel does not have.
The IPO is only network dependant at the multi-site level, as it is a hybrid system not pure IP which from that standpoint removes local network issues from effecting the IPO. It has the same VM server failure issues, but loses mailboxes as well when it goes down, although in most multisite scenarios if the P2P goes down calls still reach all VM facilities as normal. The IPO has more features, but cannot compete on the enterprise level expandability scene anytime soon. Although, as an enterprise system level Avaya has other systems as options with greater feature sets, which shoretel does not.
I have not confirmed this, but from what I have been told so far in this calendar year alone Avaya has implemented more IPO's than shoretel has implemented its systems in its entire history. From what I am told on the numbers of systems installed, the number of members of the shoretel forum 302 members VS the IPO forum 9,681 shows that a higher percentage of shoretel techs come on here than of IPO techs.
Oh, IPO VM is a pay for ports, which means how many simultaneous people can be accessing VM at the same time, including checking VM via phone, leaving VM, interacting with an AA, or hearing a Q message other than MOH/IOH. The number of VM boxes, AA's, etc. the VMPRO server can handle is a matter of server resources for the most part, not constricted by licensing. I have done 485 total which I recall, maybe more.
I think from my own limited experience that they each have their niche, or they would not both be selling. It is really not fair to compare the two as most small networks cannot support VOIP, so a pure IP system cannot be implemented without a serious network investment which changes the total cost of ownership. Also, Avaya licenses as a rule are assigned based on first come first serve, so when one logs off another can use that license.
Most of my knowledge of the shoretel is from working on some shoretel projects with a shoretel tech, and a tech being trained, neither of which had the experience to do anything but programming. They did have some issues that required some hours of support from shoretel, and the tech doing the training had advanced training, and certs. They just didn't know the punch down color pattern on a cable, if you know what I mean.
