Star Wars.
Iron Man has so many suit variants so unless you have a strong affection for the Iron Man franchise, stick with Star Wars. As others have posted, Star Wars figures are cheaper and more available to find.
Also, this is not a cheap hobby. At first, pre-ordering $200 here (like Leia) and $290 there (like Rey w/ BB8) does not seem too big of an expense. But when the figures are released and you get your credit card statement at the end of the month, that's when reality sinks in for your wallet. But I get a lot of enjoyment and a sense of nostalgia seeing the figures on display, so it's worth it.