The biggest advantage is having the extra octave when using a split, bass sound in left hand and lead sound in right.
My last to two keyboards have had 76 keys, verses the 12 other keyboard I have with 61 or less (49) keys. My Roland Fantom X-7 has slowly become my favorite.
Note: there are performance differences between various manufacturers & models. So go to GC and try before you buy.
Unless money is the issue, I would always buy 76 keys now. I dont really like most 88 key keyboards, they try and immiate piano feel to much for my liking.