I'd love to default to "they're both good, don't make me choose!" on this subject, but I'm afraid I'm going to plump for Saturn. A number of reasons:
- The definitive (at the time) Sonic compilation was a great hole-filler for families or individuals who had needed to sell their Megadrives to upgrade to Saturn. I craved a similar (i.e. Mario) collection on N64 to replace my beloved Mario Allstars/World, but it never happened. Regardless of how good Super Mario 64 was (and still is) it left a hole.
- Possibly the best digital controller ever seen, once the UK got the Japanese design. PlayStation had a curious (and in my opinion, counterintuitive) 'the d-pad is four buttons' approach, and the N64 controller took some understanding -- I grasped it immediately, many others still can't work it out. I can't empathise, but I can at least understand.
- Arcade games. Sega's main strength here, and I'll happily defend them as not being 'shallow'. They have depth! You seriously can't tell me that learning the courses in Sega Rally well enough to determine a perfect racing line and then acquiring the skill to make that theory work in practice doesn't take the same dedication and time spent that, say, an RPG would.
- On the subject of depth, fighting games. Whether 2D or 3D, Saturn offered "AAA" games of both types (Street Fighter Alpha 2 and Virtua Fighter 2 being literally the favourites of my teen years -- and anyone who claims either to lack depth really needs to revisit their dictionary). N64 had the fantastic Rakuga Kids, but little else. Mortal Kombat? Psh! Ultimate beat out Trilogy and 4 easily, which is why it had been an EVO tournament game for years.
- First-person shooters. Alright, so N64 got Quake II and Goldeneye and both were pretty amazing. Saturn offered a far better version of Duke 3D and a (barely) more faithful Quake. And that before including my own personal favourite, Exhumed -- which did Metroid Prime's schtick well before Metroid Prime.
- RPGs. Where did Final Fantasy go, Nintendo?
Although in seriousness, Shining Force III, Panzer Dragoon Saga, Mystaria: The Realms of Lore and (although this is just personal taste) The Story of Thor II gave much more than any non-Zelda game on N64.
- Wipeout 2097. With analogue control. And I *liked* that soundtrack, damn it.
- Concurrent with my initial point, Sega were more respectful of their back catalogue than Nintendo in this era. Sure, Smash Bros. onwards saw them pick up the slack, but Sega had the 'Ages' collection, Sonic Jam, shoutouts to older games in Christmas NiGHTS and Bug! among others.
- Third party content. And I'm talking the LACK of it as a strength here, which may seem incredibly backwards - but hear me out. EA famously dropped the Saturn early enough to spare Segaphiles such gems as the continual dredge of FIFA games (I believe the last we saw was '98), and a lot of third-party publishers never showed up at the party, not even late -- so Saturn owners missed out on a fair few decent titles, but at least we probably missed out on more shovelware, too.
- RAM cartridge was obscenely-huge. Unless you're playing Hexen or are a Sim City 2000 addict, you'll only ever need the one.
Of course, Nintendo 64 had some amazing things, too. F-Zero X is one of my favourite racing games, Ridge Racer 64 was tons of fun and I've actually got all 120 stars on Mario 64 about six times; I love the game that much. And it got an infinitely better version of Doom, but still. Saturn wins by an inch, but it's still a win.