In terms of physical dimensions, the 'standard' and 'high capacity' cartridges are identical. They'll both work equally well. However, the 'high capacity' cartridge will contain more ink (and therefore last longer).
In general, high capacity cartridges offer better value for money than the standard ones. (e.g. you might get twice as much ink but only pay 50% more).
If I was buying online, I'd consider using the remanufactured high capacity cartridges from InkCycle. They're not the cheapest that you can buy but they're 'full to the brim' with excellent quality ink. InkCycle is also an extremely reliable company. Cartridges seem to be in the post almost as soon as you order them:
http://www.inkcycle.co.uk/inkjet_cartridges.ph p?cPath=247
Chris