Offline as you can actually deploy some skill - online play generally just involves playing basic strategy to grind out a bonus. There are some exceptions, but by and large that's the case.
Definitely prefer brick and mortar casinos to anything online. I do not play any online blackjack, after seeing a few of my friends get stiffed on withdrawls and I am skeptical of the lack of regulation/ease to which these games can be set up to harm odds. Also, I am extremely nervous about any surveillance that can be utilized to spot APs.