What you need to find out is how these dogs react to humans not to each other. I have a female lab who is very aggressive with my older male dog but when it comes to training she is what I would consider a "soft" dog. She requires a very gentle hand in training. My male on the other hand, though he seems shy towards my female, is actually a very bold dog and required quite a firm hand while training. If I were you I'd ask to borrow the dogs one at a time for a day. Take them out individually and do some basic obedience with them and see how the dog's react to you. If you are new to training I would stay away from both a shy dog and an aggressive one, they require a lot more patience in training and can take longer to train. Try to find a dog with a middle of the road temperment, if your friends dogs don't work out you can always visit a local lab rescue.