I have both the Viggen and Av-8B, I can't speak for the Viggen as TBH I purchased it and have not spent a single second in it. My interest in DCS died off well over 2 years ago in which I stepped away and really only came back to bash VEAO when something interesting was taking place.
In recent weeks, I downloaded DCS again, purchased Persian Gulf and the harrier during the sale and spent most of my flying time in that aircraft, Although it is not feature complete, it is an enjoyable aircraft to fl. Read though the manual 1st and decide for yourself if you wish to purchase it
or not as the features no yet complete is in handbook available
here Somehow I don't think that manual has been updated as it states "only simple radio is available in early access" which quite clearly is not the case when doing the refueling training mission.
I have heard many good things about the Viggen, I heard it was practically feature complete on release but the updates since then and over the last 12 months have been slow. Mainly because of time being spent on the F-14.
Don't take my word for it, that is just from what I heard on youtube reviews and scrolling through posts on the Viggen on the other side of the hill at ED.
Harrier is an aircraft i'd re-buy again if ever required.