know you have lots of replies already, here's my take. Used the Avent manual and Medela symphony (rented) for my 1st baby practically from birth till 9 months or so. Have to admit that electric, being a duo pump is much quicker and less tiring on the hands. However the Avent pump with the silicon padding on the funnel bit, actually is more efficient in clearing the breast. Hence, when you start to pump less, will be less likely to get blocked duct using the Avent.
Now with baby no. 2 (4.5 months now), I actually bought the Avent Duo electric pump, found it has the advantage of a dual pump and the good design of the Avent manual. I figured that if I plan to carry on as long for no. 2, then buying it works out to be same, if not cheaper, than renting.
Finally, for the amount of milk you have, although differing in people, if you do not have enough, can always supplement with formula. Seems it's better for baby to have some breastmilk (as much as the mother can make) than none at all. Good Luck!