Little Darwin

I was going to jump in on the 9 speed being old technology comment, which has almost been beaten into the ground... who am I to buc a trend?

All indications are it is slightly more prone to wear due to narrower chains etc. Of course 9 speed is supposed to wear a bit quicker than 8 speed too (before that, I think the same chain width has been used since 5 speed)

Asw for the steel versus aluminum... there are flexy frames made out of every material... a flexy steel frame will be flexy, and a stiff steel frame won't be.

Also, age and riding style of the salesperson will influence a suggestion. For a casual rider, I don't think there is a frame with enough flex to be an issue other than real high end bikes that won't take well to a heavy person. Unless your intent is to sprint or climb hard, you will probably never notice you are riding a frame that a hard rider would think was flexy.

I would add a vote to find the right shop, and then work with them. They'll be able to get you an appropriate bike, and the little differences you feel between one brand and another on a short ride may or may not be significant... if they are significant any good shop is likely to be able to get you close enough that a different stem or other bolt on component could get you into the groove. When you find the right shop tell them which test ride you enjoyed most, and why.