Probably Jiraiya. Orochimaru never seems to train anyone just to improve them and out of teacher's affection, he always has to gain something out of his labors, which would be either taking my body or making me be his slave - both being a no-no for me.
Although the results from training with him would be shown much later, Jiraiya would be both a teacher and a friend, not to mention buying me ice cream and taking me on his ''research'' trips. Yeah, I'd go with Jiraiya, unless I'm extremely determined to get stronger fast.