Neither shield system is perfect, in my opinion. I think 64 has too much shieldstun (even though I love shieldbreaker combos, lol) and I think melee might have slightly too little. I like how the little shieldstun in melee gives you many quick options out of shield, but you guys really overstate the power of shieldgrabs in melee. When I first started trying to play melee I felt exactly the same way, "I HIT YOUR SHIELD AND YOU GRABBED ME BEFORE I EVEN HIT THE GROUND WTF!," but then I realized it's just much more difficult to shield pressure in Melee and oftentimes not worth it. Spacies can shine-aerial and stuff but even that has tiny holes that you can buffer rolls out of or shine OoS if you're a boss.
The way the games deal with shielding is just different, I think they're both fun, just don't try to use one games tactics when shielding in the other; picking up Melee will mean you have years of bad habits from 64 to dust off, I'm still working many of those off myself.
Koro's right about shield size for some charactes, notably Game and Watch, and Peach's dsmash is ridiculously good at stabbing shields, don't hold down against that while shielding, lol, I always get stabbed -> 3 or 4 hits from that trying to shine OoS.