I agree that Wesley is a better researcher and his relative youth to Giles meant that he had more potential. Generally, I'd rate Wes as more naturally intelligent. Even more than Wesley's youth, I always felt that Wes ended up being more open-minded than the "traditional Watcher working with just his slayer with dusty books and magic". Wesley embraced science and computers to point that he could intelligently debate physics with Fred, revolutionize demon studies by writing an article on an alternative genetic code for demons, and ably use the computers in Fred's super-high-tech lab in S5 to productive ends (i.e. diagnosing Illyria's radioactivity and building a blaster to zap her energy). Giles was more open-minded than your standard Watcher back in England. However, he still stubbornly clung to mysticism and refused to learn scientific subjects or use a computer competently till the end. For that reason, I do think Wesley has a stronger "right-brain"- more analytical, more gifted in math and science, a more organized and logical brain. Giles frequently appears disorganized.
Both guys have an excellent command over language, they're very articulate, and they're both thoughtful to an equal extent, but Giles's wisdom honed by age allows him to stand-out.
I actually think Giles may be more gifted than Wesley in social skills and knowing how to handle people. However, Giles mostly only focused on his social gifts on Buffy and had very distant, non-meaningful relationships with the others. Meanwhile, Wesley worked harder at being social and earned close relationships with characters as different as Fred, Lilah, Gunn, Angel and Cordelia.
It's hard to judge in terms of magic. Giles demonstrated more magical strength and know-how through the series. However, Giles had a whole misspent youth where he worked out his magical abilities without the control of the Council. IMO, Wesley rigorously followed Council orders until the end of BtVS S3 so he never had the independence and free-form to really learn how to do magic as supposed to suspecting magic. It took him until AtS S5 to really start diving into magic, bolstered by his developed confidence and the confidence of having an entire W&H mystical team and tools to do advanced spells.