I guess the only way to know their track record is to ask them? But I second MrsG - go and get a property magazine, find a few similar houses to yours that are being sold by agents you might be interested in. Go to the open home and see what the agent is like, and also compare the ads to what you see at the open home. Have they made the effort to identify and advertise the strong points of the properties?
And also I think its important that you like them and feel comfortable that you could stand up to them. When we sold our last house, DH ended up being in Aussie when we got the offer so I had to do all the negotiating. The offer was quite a bit lower than we wanted, and the agent was a bit pushy that we should take it. I stood my ground and we got a deal we were happy with. I was confident that the agent would remain professional if I refused to do as she suggested, so I wasn't afraid to make sure it was a deal I was happy with.
Both times we've bought a house the vendors agents have been terrible. I'm sure the only reason we could afford this place is because of its terrible marketing that meant most people didn't bother coming to look at it.
As for the lady keeping in contact with you - I'd perhaps discard the two who didn't, but don't let that put you off trying out others. I did something similar, only 2 agents from competing companies kept in contact, and then when we decided to sell, I rang one and signed up, only to have the other one ring to "catch up" that night (I suspect there must be some kind of property grapevine!). I did feel a bit stink saying sorry, we signed to someone else today, but you've got to do whats right for you, and I haven't had anything to do with the other agent since, so its not like it has actually mattered.
Good luck! Selling a house is more stressful than buying one, I think