My experience with both Solidworks and Alibre/GM is Solidworks is a better package. That is why Solidworks commands the higher price. Solidworks is more stable, better supported, more widely recognized in the industry, and has more stable integrated add-ons.
At my day job, we use Solid Edge and sometimes we use Solidworks. For our application, both are superior to Alibre/GM. However, for my home machine shop job, I use Alibre. The biggest reason is price. I'm willing to put up with less accessible tech support, fewer add-ons, and less developed features for (what used to be) a huge price difference. In September 2010, I paid $499 for Alibre Design Pro which included sheet metal, Keyshot, and the first year of maintenance. Now, the equivalent "level" software is $1299 and sheet metal is gone.
Further, in August 2011, I paid for another 15 months of maintenance for $299 as Alibre Design Pro was working great for me. Then came the automatic update in late 2011 to upgrade to AD2012. And it quit working. I could not open any files I had created and could not create new ones. Went back and forth with tech support for 8 months and was finally told it wouldn't get fixed. I spent countless hours uninstalling and re-installing AD, trying different video drivers, checking for updates for anything that might be causing problems, etc. My only solution was to buy a new computer, install AD and wait for tech support to re-assign licenses. Solidworks would not have put me through all that. But I spent considerably less than the cost of Solidworks.
If I had employees going through the situation I went through, Solidworks would have been the less expensive package by far. However, I presume my situation was not the common one.
My opinion is that it really comes down to price vs. risk. Solidworks is essentially no risk but you pay for that. Alibre was much less expensive but was a little buggier and had less tech support. For what I paid in 2010, I really like Alibre.
GM could reach some places SW cant.
