The brand is irrelevant.

More important for you is to decide whether you want to
- Road race, or go fast (road bike with U-shaped drop bars)
- Just use it for solo training and errands (hybrid or commuter or road bike with flat bars)
- Mountain bike or off road (mtn bike)

And fit is king. Do NOT settle on a bike that 'almost fits.' That'll kill whatever quality you spend on the bike, but most salespeople will push you into buying an 'almost fitting' bike.

In bikes of a similar class (road vs road or mtn vs mtn), it's 99% rider ability, for real. Pros routinely switch bikes in and out, even in the middle o f a ride, with no problems.