Too short a trip to split, especially if this is a first trip to Bali. I'd spend all 6 days there, maybe 3 in Ubud and 3 somewhere else (pick a coast). If you have been to Bali before, maybe skip it or transit directly to the Lombok ferry, then play the return to Bali by ear according to when you get tired of Lombok. You might need at least one night on Bali depending on your outbound transportation schedule.