DS2 has just finished at Winchester we chose it because we felt it was a unashamedly very civilised intellectual environment, academia is always going to take precedence over everything else. We also felt it's not a desperately competitive environment, my DS is not competitive. After he started I saw a programme about Eton the beaks (teachers) read out marks and your place in the class, that I understand rarely happens at Winchester. Boys are very much allowed to be themselves there is no pressure to conform, my DS was a quirky lone wolf a true non conformist and he has remained like that and is comfortable in his quirky lone wolf non conformist skin he found many kindred spirits. Music and drama are either an option you choose to do from day 1 in the case of the former or an extra curricular activity in the case of the latter. I know some parents who've said their DS's think there is no "fun" at Winchester my DS has not found this. I also know quite a few boys who were considered to be "very bright" at prep school who've got a bit of a shock at Winchester and at the end of the first year are in the "6th" set for the everything and feel rather demoralised, there are a lot of exceedingly bright boys there. Another said her DS found it too "studious". Also at Winchester your DS's HM is the most important person, ours has been simply fantastic but if your DS doesn't get on with his I can imagine this would be difficult.
Winchester currently only offers the much harder Pre U it is significantly more work than the A level and it is much harder (my DS could get an A on both the physics and maths A level papers at the end of yr 11 and was advised he'd be bored stiff doing AS French) and the boys do wonder why they're being put through it although none of course know how the A level works, I understand it's much more bity less joined up and more prescribed Wykamists are not always good at prescribed. As we know the pasture always looks greener etc. Winchester A equivalent grades are significantly better than Etons and it appears that more and more universities are asking for A's.
There is no compulsory sport after the first couple of terms and no "major minor" sport, my DS did what would be a minor sport in most schools three days a week from day 1, no team sport player he threw his football boots away at the end of the second term with glee "no more team sports ever again!" I dont know how good the tennis is but I believe it's all grass courts but other may know more, you could look at "news/sports results" on their website to get a feel for it.
I know some feel Winchester is more nurturing, and it's very liberal and forgiving which I like adolescents are biologically programmed to take risks and schools need to be more forgiving IMO, it's also good for the disorganised; there's set times for prep etc, I've heard quite a few disappointing things about Etons pastoral but I could probably say the same for Winchester.
My DS who's boarded said nice yr 2 only got a single room in yr 13 he didn't like it, he genuinely believes that friendship are made when lights go out. At least 3 mums I know said their DS's have really struggled to make friends at Eton but this may be their personalities rather than Eton it's self. Winchester is sending more and more to the Ivy League with the associated expense I don't know what Etons doing.
Finally Div is unique to Winchester, for those boys who love it it's what makes their school experience, others feels it's a waste of time "learning and its associated prep for the sake of it". My math/science orientated non reading DS loved it and studied in particular literature that I know he wouldn't have at any other school, there is no history/Eng Lit IGCSE so the dons (teachers) who teach those subjects have more freedom to teach what and how they like). A friend teacher English at another school he said he'd love to be free of the examination curriculum and teach how what he wanted too.
I. The parents at Winchester are a casual bunch, if you want lots of social activities/opportunities I wouldn't recommend Winchester, as a review in Tatler many years ago said it's not the place for the "interfering parent, the rugger bigger or the social climber". In my experience from MN and friends many Eton parents think it's the best place in he world and can't or won't see anything wrong with it, they have completely bought into "Eton the brand" maybe I'm an embittered old cynic but I personally don't find this very healthly, no where should be or is that perfect, Winchester parents are IME are able to see its strengths and weaknesses warts and all but still believe in what it offers.
Good luck to your DS wherever he goes.