The new ShoreTel phone will be nice - I saw a pre-release version at their factory back in early March. However, you can do a bit more with the Mitel, it has more keys/buttons, works with the wireless handset, and they give you the built-in HTML toolkit for customization. Of course, the ShoreTel set will support the G.722 wideband codec, which I don't think is available on the Mitel.
If the remote users will be taking customer/business calls from the office telephone number, then I like the ShoreTel using a separate line to complete the call (we use it that way now). If the remote users are going to use a local line for most of their customer calls, then I think the LIM is a better way to go. I have found too much call quality problems relying upon an unmanaged internet connection - most of the time the call is good, but variables (such as congestion with your local internet access provider) will be detectable. If the soft-phone calls are primarily from within the company where an occasional lower quality call is ok, then it won't matter as much.
I like the ShoreTel PC tool (the Personal Call Manager) better than the Mitel PC tools - so if your users are PC centric I might favor the ShoreTel but if they are phone centric I would favor the Mitel. If your users are OCS centic then I favor the Mitel
I hope you don't expect to spend $80K to $100K for a system in the size range you mention. It should cost less than $50K unless you are ordering full call center features or a web-based conference bridge.
I have used both, and our clients have ordered both systems with success. I don't think you would be unhappy with either one (assuming the local vendors for each are equally competent). Wishy-washy enough for you? :)
If you read all of my comments, I sincerely would base my choice on the factors I mention -- if you spot a match with your situation, then that would be how I would recommend.
