I think, as some have eluded to, that the benefits depend where you are originating from (for me, LHR) and where you fly to most regularly (Africa, Asia and Europe mostly). My main two memberships are Oneworld with BA (currently Gold) and Star Alliance with LH (currently Gold). I was silver with Skyteam (KLM) for a while but it had little benefit other than business class check-in and fast track boarding (you had to pay for lounge access!).
For me, Star Alliance Gold has some very important and good features, but also some issues: that you get two years membership on top of the year you qualify is great and a significant benefit compared to OW. Also since my short haul is in economy, the priority boarding is very important so that you don’t get on the plane when there is no cabin space left. Gold compared to sliver, now that the new T2 is open at LHR, you would get lounges on three of four legs if you are flying on a connection with LH or one of the M&M partners (but don’t get the priority boarding) so there is not so much difference between gold and silver (particularly since lounges in destination airports in developing countries are often not of much value). Also the half price partner awards is very good. One of the big things I don’t like about LH M&M (SA) is that you don’t get any additional bonus on you miles (status and tier) for being gold compared to silver. There are probably many more features I have missed but these are the most important to me.
For Oneworld, the bonus mileage accruals is now fair and better than SA (they have now modified it so that gold is differentiated from silver). I think too many people have priority boarding with BA (Gold, Silver and Bronze) but I tend to fly long haul where cabin baggage space isn’t usually so much of an issue. As with SA I think the lounge access differential is quite significant in OW between Silver and Gold – I would hate to have to use the Business Lounges in T5 again, they seem to be constantly downgrading the food and beverages, and are overcrowded particularly in A terminal. BA also has the Amex Card which is generous in terms of miles (particularly for BA bookings) and the companion voucher which is invaluable. SA doesn’t have this benefit, but as above does have the half price companion price for airmiles bookings.
Generally it appears there is better reward flights availability in premium cabins on SA than on OW (this is LH compared to BA).
On upgrades (complementary) probability correlation to status, it’s difficult to tell. I have had a lot with BA (15-20 in the last 7 years) but only one with LH. Again this is affected by routes.
So, a difficult one but on balance I think I would rather put effort into retaining gold on SA that BA.