I would suggest you apply for green card process while ur parents are in Nepal. If everything goes fine, they will be with you within 5-8 months as permanent residents.
If you invite them on visitor visa and apply for their green card shortly, USCIS might consider that as visa fraud as they got the visitor visa with non immigrant intention and applying for green card within short time of arrival shows the immigrant intention. If they come on visitor visa and you apply for green card processing after several months, thst might be fine but depends on USCIS.