Wow, this is a very good question and is a tough one to answer --- here's my opinion: Wellness is a bit better as far as not being seen by passing cars on the road --- it's a bit elevated and a bit further back from the road vs. Hereford, which is pretty much at a 4-way stop sign on a small road right beside a large road and right down from a Turkey Hill. Only weird thiing about Wellness is that there is some sort of business attached or adjacent to it a little further back from the road (whole complex looks like a former, old 1-story motel), and the workers at times come out in the parking lot for smoke breaks --- one time I walked in and felt the stares of 3 fairly hard-looking smoking non-Asian women. At Hereford, you can park right beside the (front) door and run right in, and I've heard of people parking at the Turkey Hill and walking over (so their car does not stick out like a sore thumb).
As for cleanliness, I've found both places well-above-average and at times impeccable. As for friendliness, it has varied with the mamasan --- I think there was a real crab at Wellness for awhile, but that may very well have changed. Never had an issue at Hereford, but I heard that a mean one came and went quickly before the last closing. Talent varies but has always been above average inmy opinion. Check this board and others for up-to-the-minute commentary.
I've always felt best in calling ahead and asking who is working (they won't usually tell you but will confirm names if you give them to them) and asking if they are busy --- you'll be asked "when can you come", and indicate to them when you'll be over. It's not really a formal appointment, but when you walk in, Mama knows you're coming.
That about covers it for me --- I'm sure others have some opinions.