To be honest at the suite level it isn't a like for like comparision. Aspen has specialised its product range over the years by diversifying from a data historian base into industry specific simulation and mathematical analysis tools. As a user of IP.21 one of my biggest concerns was that Aspen was concentrating on the diversification at the cost of losing focus on the historian.
The difference I see with OSI is that they have tried to stay true to the core historian product; partner companies have tended to do the specialist elements (which may or may not have been marketed via OSI).
If you are making a comparision then it needs to be done at the right level, which means knowing your requirements so you can ask the right questions. Typically you might buy a pre-packaged solution from Aspen for a particular set of requirements, whereas you would probably buy the building blocks from OSI (if an off the shelf solution wasn't offered) and build the solution yourself. Again swings and roundabouts, pre-packaged might look good but how flexable is it to adapting to the way your process works.
The other thing to consider is that the likes of the Aspen AMS or AES suites contain many products most of which require their own licenses (and costs) in order to use. In the manufacturing environment I've only ever seen a few of the many products available on the AMS disks actually used - typically IP.21, Batch.21, Web.21, SQLPlus, Process Explorer and AspenCalc of which IP.21 and Process Explorer must account for a good 99% of the usage.
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