Hmm. It's a good question, I suppose. I have no affinity for feet and tend to avoid looking at them. When it comes to tickling, I'll go for them if available, but I don't make them a prime focus. However, boobs tend to scare me, just as many naughty things do, so I usually avoid looking at them as well. A girl I knew once remarked I was the only guy she knew who looked at her face instead of her breasts, in fact. But that's getting away from the topic. When it comes to tickling breasts, I have so few opportunities I wouldn't really know.
I'll say this. For looking, breasts, as they're certainly more alluring for me than feet, even if frightening at the same time. For tickling, definitely breasts. There's something of an allure of forbidden regions there, so be sure. I'm not often a 'ler, but when I am I want to see the 'lee's face and reactions up close. Going for the boobs, I can be right near her face and enjoy every laugh, growl, or funny face that she produces.