I learned Italian as an adult and now speak it fluently. I have been learning Spanish for about a year and have made a ton of progress with much less effort by leveraging my knowledge of Italian. I do not mix them up, and i feel that this is mainly because I already a very good core in Italian before I started learning Spanish. As far as one being more useful than the other: in my geographic location of the world Spanish has a lot more utility than Italian because of the large number of Spanish speakers here, (although French and Italian are considered to have a bit more "prestige", if one cares about such things). Personally however, my life is geared in such a way that Italian is much more useful to me personally, knowledge of Italian gives me more professional and personal advantages than Spanish.
At any rate, that is just my personal experience,
In bocca al lupo!