I can give you advice about italian becouse I'm actually, italian.
First thing: italian is difficult. It has loads of irregular verbs, and they don't follow any kind of patterns. Looking at the attempts to speak in italian by english speakers, I think this is the biggest difficulty. Even spanish has many irregular verbs, but not quite as many as italian has.
Also, spanish and italian vocabulary is really really similar. I mean, if you speak italian in Spain, almost everybody understands the basic meaning of what you are saying without effort.
Now, spending a words or two in favour of my language:
I love the way italian sounds. Maybe is just because I listen to it all day, but italian poetry seems to me the most beautiful. Way better than spanish or french
And we have opera! I can't imagine what it's like listening to opera and don't get what it says (even if sometimes it's hard to understand everything even for me... xD).
If you are really so much into language learning, italian could be a good choice. And after having learnt spanish and italian, learning latin will be a breeze for you!