Which one would be the beat as a thief/archer who uses magic on the side? If try would both be terrible, what would be a decent alternative?
Of the 2 beast races The Khajiit recieved a bouns +5 to sneak, secrurity and light armour as well as having the abilty Nighteye (good for helping see in caves whilst sneaking)
The Argonian has a +10 to security and +5 to Illusion and mysticism (useful for the spells chameleon, invisibilty and detect life)
.... These stats are from Oblivion as no news about what each race for Skyrim has in the way of stat bonuses, so can't give a recommendation as to which is best. I would probably pick the Argonian though
P.S I played as a wood elf in Oblivion and they have +10 to marksmen and sneak as well as +5 for light armour and alteration (which has the unlock spell), but that is off topic since they aren't a beast race
The beauty about Elder Scrolls games though is that you can make any race as good as any stylled character you want, so starting stats are just that. They can be completly unimportant later in the game as your character levels based around how you play. So it really comes down to which race you prefer more?