My biggest needs are ease and dependability of starting (painful shoulders). I also want enough power for thick weeds.
It would probably help if people knew the size of the jobs you were going to do. If you are going to start it and wack for half an hour that is one type job. But say if you were going to run it 5 minutes on a job then let it sit two weeks, then do 10 minutes in the garden once a year that is another. I went for battery because my jobs are 3 minutes once a week. Someone with a large lot who is out a couple hours a month that would not work. I do knock down a 3' tall winter crop in a 25' x 15' garden two or three times in the spring, and the battery powered one is fine for that. But if I had an acre or two it would not be adequate. However with your bad shoulders a battery powered unit should be a consideration if the jobs are small. Make sure whatever you buy, parts are available. Some of the wackers these days the only way to repair them is to replace them.