Of these two, both members of the "München Sechs", I would choose the Spaten. My favorite "helles" from Munich is Löwenbräu and my favorite beer tent at Oktoberfest is the Hofbräu tent.
I don't tend to drink much beer when I'm back home and with nearly all things in life, atmosphere and presentation is 90% of the pleasure.
I'm assuming you're talking festbier since Hacker Pschorr sells other brews as well. I've never had Hacker Pschorr, but yes it's that time of year. Marzen for everyone!
Spaten Oktoberfest is tough to beat, even with Hacker Pschorr being so good. Another one to consider if you're trying new brews is Ayinger. Accidentally found myself in their Munich location, it was my first time hearing of them, they hooked me.