I buy both but I really love one main theme done indepth like the gps offer.
If for some reason a gp and an ep came out at the same time - I would buy the gp first as I love the degree of depth and single subject the gps focus on over the ep that throws a lot of sometimes random and less thought out items. I never feel like a gp is missing something - in fact I generally am amazed at how deep it covers it's theme - where as the eps are nice but it's like the overall idea of the pack sort of get lost in all the directions it spreads out in. Like CL for instance - it is supposed to be about living in the city - but I have to ask myself - where is the actual city these sims live in. Not a city store to be found - not an office building or town hall - no police or fire department - where the heck is there a city like this I ask myself because in truth it is an apartment living pack surround by tall buildings we cannot venture into. There is no city as we know cities. This bugs me to no end as nothing what makes a city - a city - exists in the ep called City Living. It has a tiny aspect of a city having a festival now and then but other than that how is this City living?
I feel if it had been a gp called The City - we'd have actually gotten a city....