Depends on whether you're more into the little Coopers and a few bikes or a few Coopers and some proper old racing cars! Personally, if I was only going for one day, I'd go on the Sunday BUT it's more expensive. If I had the choice of either, I'd check the weather forecast and choose the driest day however, there looks to be little to choose between them as far as precipitation goes this year!
Other tips? Get there fairly early and catch a bit of the morning practice action on the long straight shortly after the start. As soon as the practice finishes, grab a burger/pork bap/insert food of choice (!) before the queues get too long (get Mrs Western or any companion to grab coffees while you get the food) and wander around the paddock area while all the cars etc. are there (great close ups). Before the afternoon action starts, head up through the woods to the hairpin before the straight just before the finish - decent panning shots through the trees, a good chance of a few low speed spins, you'll be VERY close to the cars as the head up the straight so you'll get a good fix of "R" and open pipe rort! (Earplugs might be a good idea...) Oh, and pray that the Napier Bentley is there - fabulous sight and the noise...
I'm probably going on both days since it's not too far away from home - look for a well padded gentleman in a rather disreputable Tilley hat held together (the hat, not me!) with a TP patch. I know there are a few people from here who sometimes turn up.