As a salesperson who sells both products, I'd recommend aiming toward the Sennheisers as their build quality tends to be of higher standard than Skullcandy, which I have countless pairs coming back to my store broken through regular use or abuse within 30 days where I can't recall seeing a single pair of even low-end Sennheisers being returned with a similar issue.
If you have serious doubts about what you would like more, I'd recommend swinging by your local big box store, like Best Buy, trying the headphones, returning them within the return policy (there is no restock fee if you have all included accessories) and then making your decision based on the time spent with them. If Best Buy doesn't have them on sale, then I'd recommend looking elsewhere for the final purchase as they usually sell headphones for MSRP.