Impossible to say which will be best for you. Such a personal decision. As Anna mentioned, you can always get a rolling stool to use with the long arm, if needed.
As for machines, check out the Juki 2200QVP sit down machine. Some things I really like about it is the fly wheel is within easy reach to pin-point dropping the needle or taking up the bobbin thread.
Other health issues ... sore feet perhaps? If you have a frame set up on a hard floor, a long line of cushy mats will help. There are also issues (for some) with crawling under the machine to check bobbin tension, but mirrors or a camera can help.
good luck whatever you decide on!!