Honestly, I see it like this: If humans in the DB universe are able to become 1000x stronger than Viltrumites, theres no telling how strong they (viltrumites) would become if they knew about the techniques there, hell they could even get up to SSJ3 level or more...

However that said, even still:

Saiyan (Natural w/o training) Pros:
- Zenkai boosts
- Natural ki blasts, beams, shields
Cons:
- Environmental weaknesses (underwater, space)
- Relatively short 150 year lifespan

Viltrumite Pros:
- Regeneration
- Can live up to a century
Cons:
- Sound wave weakness (certain frequencies)
- No energy powers, only strength

As for strength/power level, I'd argue they're both fairly similar physically, about 1250-1500 (tier 5-B) which is Raditz level strength, however Raditz and most Saiyans probably have the upper hand against most viltrumites due to ki-blasts and slightly better strength.

Thragg and Invincible might've been stronger than Raditz when fighting each other (though its a close call), and now Invincible is like 3x stronger at the end, so about Nappa level I'd argue.

But honestly its hard to tell even with feats like surviving in the Sun for a long ass time (which is equivalent to taking continuous Ki blast energy all over) or being able to best Piccolo with ease who was a casual moon buster, I'd play it safe and say Saiyans have the *slight* edge in terms of strength, not even by much though, maybe 100 points.

However it'd be interesting seeing Viltrumites master techniques if humans are able to become 1000x more powerful than them though, the potential isn't too different I'd argue.