WD Live which is using Sigma chipsets which many claims the picture is sharper wor.....

Depends a lot on the pairing of the player and the TV. Both player got slightly diff colour and brightness/contrast  schemes.

So if Realtek default settings happens to suit your TV better, you will say Realtek is better. If Sigma default suits another guy's TV better , then he will say Sigma is better.

So subjective, the only way to get your answer is to borrow both the player and test it out yourself  .

I recall reading somewhere that the mini does not have UPnP feature unlike the POHD Big but will be made available with future firmware updates. Any update on this?

Am considering getting a mini for my living room as I really don't enjoy moving my current POHD Big in and out from the bedroom

With more and more people moving towards have bigger screen and better download bandwidth, I notice many places are now offering TVB series at 720p resolution.