the general rule is that theory is harder to get into... i just see in the theory people here. They have 1 grad student per professor maybe 2, while the experimentalists run around with 3 to 5 or even more depending on the project.
Any research experience is good. May it be theory or experiment. Experiment is more "appropriate" fir the undergrad because you can do a lot more in a lab per se rather then converting coffee into formulas, so it shouldn't really hurt. The fact that it is simulation work shows that it is more theory oriented anyway. I hope you understand what you are telling the computer to do.
The problem lies in that you usually need a strong mathematical background and most undergrads don't have that till the very end of their undergrad, i.e. Analysis, Advanced Linear Algebra, etc. I am auditing a grad math methods course at the moment and all the entering theory students are in it. You can see that even before you start they will first put you in that before you can really start working.