My new car – cha cha cha azure 1. Pop (only had him 5 days) has tan leather seats – definitely the more retro choice – and I am very very pleased with them.
However I think the Lounge seats are very nice too so I can see your dilemma. I saw the white with black check cloth seats on several cars in the flesh – I think they're really lovely but they're quite light coloured and I imagine these would show the dirt quite easily. One I saw in the showroom looked like it had a dark stain on the drivers seat already.
If the cost is not a big factor then I reckon the tan leather seats are the best, they really are a bit special with cream piping down the sides which really sold them to me and will look great with the BNW colour car. (incidentally the red leather doesn't have the piping unless you're buying a 500c). So far I haven't driven my car wildly enough to really tell but I they don't seem slippery – the seat shape is very supportive and comfortable.
I have been waiting for it to stop raining and work to not be so busy so that I can take some proper pics of my lovely new car, hope to get some good ones up this weekend including the interior.
Also - bear in mind if you have ivory ambience the lounge check seats will have ivory piping, not black as in some of those pics.
Here are someone else's pics of tan seats in a BNW colour lounge.
https://www.fiatforum.com/members-motors/194554-our-fiat-500-1-3-lounge.html