If I have to choose between THOSE two (which aren't the only two categories of anime) I'm choosing Mecha because while I'm an avid anime lover, the romantic interests in 99% of anime is lackluster and horribly written. It's like watching the infatuated journals of school children come to fruition.
I would say both mecha and romance because there are a lot of mecha anime that have romance in it such as:
1. Code Geass
2. Linebarrels of Iron
3. Eureka Seven
4. Gundam 00
5. Gundam Seed Destiny/ Gundam Seed
I'm a huge fan of both! :D
But there's no better explanation than this because, well it's everybody's opinion!