Personally, I would not spay!
It is too big and risky an operation in guinea pigs that should only be performed if there is a medical reason that cannot be treated otherwise by alternative, much less intrusive treatments like hormone treatment or even draining. The mortality rate is sadly still fairly high, much higher than neutering boars.
Unlike with rabbits where spaying actually prevents cancer, there is no direct health benefit for ginea pigs in term of health or life expectancy.
I have had so far one girl not waking up from an emergency ovarian cyst operation in 2010, performed by a local vet, and one girl surviving and recovering well from an emergency spay due to a grossly enlarged, borderline pyometric (infected) and lump filled womb in 2013, performed by the probably most practised spaying vet in the country so she was very lucky considering her low body weight of just 700g.