Any one of the three (Legend will actually be called Magix these days if you looked at a brand new system) would feel like a breath of fresh air after a 20 years old system.
I have a feeling that Magix will be phased out by Avaya in a couple years as it's market is being taken by both Partner (lower end) and IP Office.
From the remaining two IP Office has much more functions and extended capabilities (like voice-over-IP). Its voice mail system (VM Pro) is also extremely capable, with virtually unlimited auto attendants and message storage. In addition to that it is very easy to tie IP Office into your computer network and have people call from Outlook by clicking a single button (using TAPI interface), which is a very neat feature. For a very busy office it may also matter that IP Office can take (multiple) full T1s whereas partner only takes up to 16 channels off a T1. All these features come at a premium in price, obviously, which I would estimate at about $200 to $400 difference per extension, depending on the configuration.