What will give me better sound quality and quality overall: a pair of Sennheiser HD 429 or Audio-Technica ATH-M30?
Which headphones are better?
I'll use them for music (I'm not an audiophile), gaming and entertainment.
I don't know those models. The best you can do is find a way to try them yourself, using music that you know well.
If you can't, look up as many reviews online as you can. Head-fi is a good place to check for reviews. Amazon typically has a lot of reviews (but often less detailed than head-fi).
I mean actually reading the reviews to see what people thought of the sound. Then you can try and figure out if the sound would suit your tastes.
Going strictly by the number of positive reviews on amazon, it's a safe bet that the ATH-M30 sounds good. That doesn't necessarily mean the sound signature will suit your preferences.