Thanks everyone for the welcomes and the helpful info!
After reading all these replies I can see I'm not yet any closer to picking out the exact trimmer for me, but I've gained some knowledge. That was my intent in the original post was to understand the difference in the 2 types of engines, it just happened that the reason I wanted to understand was I'm in the market to get a trimmer.
I think its going to come down to just going out and looking at a few different models, and as what was mentioned, try some out and use the return policy if needed. The last time I used a trimmer was about 15-20 years ago so I'm sure things have changed quite a bit since then.
Thanks again everyone, this is a great forum and I'm glad I've stumbled upon it.