I think IESE definitely has the reputation of being a really rigorous MBA program, and is regarded as the most pretigous in Spain.
However, ESADE is also a top school, and it has a very good reputation. It is arguably the #2 in Spain after IESE. (It's sort of a toss up with IE, but I think most people would give more prestigue points to ESADE because its a longer-standing program.)
I live in the US and would say all three Spanish schools are about equally unknown here, unfortunately. Hopefully that will change!
The difference in IESE and ESADE is definitely in attitude and approach. I think ESADE tends to attract a lot more diverse candidates from different industries, while IESE is more appealing to those in the finance/consulting world because of the schools excellent contacts in those areas, and strong emphasis on "hard" skills.
However, I think those with non-traditional b-school backgrounds actually may have a better shot at IESE. I think a higher proportion of advertising/marketing/sales/communications/industry types are applying to ESADE vs. IESE because they view it as the less stodgy of the two schools -- and more "fun."
However, IESE would probably love to have some of those ESADE applicants to balance out all of the suit-wearing types who typically apply there... so I say, apply to both! I've heard from IESE students that they make time for fun too