I faced the same situation 18mos ago. It was a one-seat purchase, and I was the one making the decision. One of the main factors I tried to evaluate was just quantity of bugs. As a long time Acad user, I knew the dirt, and was running from the chronic bugs. I had hoped that Solidworks would offer me a much more reliable platform, but saw that there were some problems. Enough problems that I, as an outside observer, could not clearly see a leader.
I eventually went with the Inventor 5.3 series, largely based on the idea that the AutoCAD 2002 program came with it. I felt this would be a safety net if I had problems with Inventor. I did have problems with Inventor, so much so that I abandoned it. The reality is that you CAN'T really know a software package as complex as this until you've used it for 6 months.
However, I've now decided to make the switch to Solidworks, largely due to the dealer making a good deal on a cross-platform upgrade. (I'm also very grateful I said not to go with the subscription to Inventor, giving me cash to play with for Solidworks)
Even though I've made the decision to purchase Solidworks, I have continued to look at things, and have some observations regarding upgrade and subscription policies;
Autodesk; They have substantial penalties when you upgrade and aren't on subscription. Evidence that they don't feel their upgrades earn their price. Subscription used to not provide any support, but recently they added some level of technical support. Autodesk does, however provide service packs even if you aren't on subscription. Another evidence that they play games with upgrades and subscription is when they removed a number of add-on tools from the 2002 version, granting them only to subscribers. Then for the 2004 upgrade, they were re-introduced to AutoCAD as a new feature. With the Inventor series, they gave subscribers a minimal upgrade version 7, and introduced the Inventor-PRO series, for which subscribers had to pay an upgrade fee. Another dirty trick.
Solidworks; Their upgrade policy for non-subscribers does not have significant penalties. This indicates their confidence that upgrades will earn their fee. I was very disappointed to learn that service packs are only available to subscribers. Until recently they were even withholding the user-contributed model library from non-subscribers. They are artificially adding to the value of the subscription. But they've always had a strong plus with the support that is part of subscription.
Often it feels like we buy our CAD software from marking companies, and not CAD companies.