I think there's a lot of factors you have to take into consideration when talking about who's more powerful, first you have the whole first born theory in which people think the first born is the most powerful which would make Wyatt the most powerful of the two, but then you also have to consider powers in general I think; Prue was the first born so you would think she'd be more powerful than Piper, but power-wise...what's more powerful then being able to freeze someone and stop them in their tracks one moment only to blow them up seconds later? So by that logic, if a half-whightlighter has to have passive powers, wouldn't a half-elder have to have extremely passive powers? Thus, again, I'd think Wyatt would win. However, Elders are more powerful than whitelighters because they're over them in the magical food chain - in that respect Chris would be more powerful.
You also have to take into consideration though, I'd think, who practices their craft more (or who's most well versed in what their powers can do - like Prue did studying that time about astral projection) because that would equal being more prepared and thus more powerful in the sense of readiness; you could also add in the factor of who's had their powers the longest if they both practice the same amount of time - just because it leans itself to practicing longer and being more prepared once again. From what we saw on the show it'd be easy to say Wyatt was the most powerful because everyone always talked about his unlimited amount of powers being a twice-blessed child, but how do we know that Chris didn't have the same and all he needed for the time he was in was orbing and telekinesis so he didn't show any other powers off? We could even put Melinda in this argument as to who's the most powerful because she'd be just a witch like Piper since Leo was mortal when she was born so she'd be open to having powers that weren't bound by half of who she is...so she could actually be like Piper and be more powerful than her siblings even though she's not a first born or someone who practices their craft all the time like Phoebe did early on; there's just a lot of factors to take into consideration I think.
Like many things with 'Charmed', nothing is as simple as "who is?", "what is?", "what if?" or "why?".
Hope this helps a little anyways though.