i rented medela symphony and have mini electric and swing at home.
the rental pump is faster, quieter and more comfortable. can stimulate milk production more efficiently. from 3.5oz to 5oz in 3 days. in case (rare), the pump is down, they change for you immediately. but you are right that rental pumps are much more expensive.
1. if you just pump at work (i.e. not "full pump" type), swing is more economical and convenient (but you need 2 to have more milk production)
2. if you want to BF for >=6months, rental pump is not that expensive.
3. if you want to stimulate the milk production more efficiently,. symphony may be better.
indeed, symphony is better than swing in the sense that it is faster and can stimulate milk production more efficiently.
i have been pumping for >6months and i rented symphony in the 2nd and the 3rd month when increase in milk production was needed. now, i'm using swing (x2). the milk production still increases steadily and slowly.