For starters, I could have a single watermelon delivered at 11pm for a party I had going on, but with Safeway, I could not do same day delivery and it would need to be delivered during working hours the following day because the other times were already booked. Safeway's delivery mechanism is probably good enough for large offices that need regular deliveries, but Instacart is good for all other times. I probably would just stick with a single delivery service for everything so I wouldn't waste my time using Safeway's service.
Instacart feels very similar to Google Shopping Express, and if you haven't tried out either service, you should definitely try one of them out. Google Shopping Express can't deliver perishables, but they do support Costco, so if you don't have a car and you need 50 rolls of paper towel, that's the best place to be.