They are both fantastic packs so you must ask yourself if certain cars in these packs you would prefer to race with more than the other pack thus you have your answer but since the packs are on a very good sale it's a no brainer to buy both if you can afford the purchase as you won't regret.
I just bought both GT3 packs in the sale and I'm well impressed with all the cars. I particularly love the C7R. Drives great and sounds fantastic. Get both.