So I'm curious whether the OP has made a decision?
Having dealt with a LOT of real estate agents in the last 4 years (took 18 months to buy new property, 6 months to sell original property x 2 agents, and sold a rental property myself on TM) I would go with the agent charging the lower %. The big guys claim they have more buyers, but I've never seen proof of that. They advertise an Open Home mainly to find people looking to buy then harass them afterwards trying to get them to view other properties. This may give them more buyers if they are holding more Open Homes, but the other agents who charge less will employ the same tactics. Bigger agencies will negotiate harder? How does that work? You think because they are potentially getting paid more they will keep going longer? It's not been my experience. Bigger agencies are very keen on auctions up in Auckland even when the auction market slowed considerably - why? Because they are pretty much guaranteed to get their commission. If they get the seller at an auction and the price isn't what they want then they will apply extreme pressure to say 'the market has spoken' regardless of whether they claimed the house was worth $25k more from their 'extensive data'!
Seen all these tactics in action and never seen any higher value from the big agencies who often have extreme pressure to simply make a sale and make their targets.
IMHO smaller agency will do as good and if you sign a short contract with them and are not happy then you can move onto the big guys and lose more money