I guess it really depends on what you prefer and where exactly your seats will be. I have sat on the sides and straight back from the stage and I prefer the sides. But again, if your seats are quite high up, neither would be that great. Still, I personally would go for the side and as low and close to the stage as possible. Obviously, the floor would be the best.
As for the ticketmaster, it will depend on the venue, but usually you just have an option to choose the price zone or on some venues the section. You do not have an option to choose your seats. Unless that's something new they have added recently.
Also, ticketmaster requires you to have an account. If you don't have one already, go ahead and create one before the tickets go on sale. Ordering tickets is timed (meaning, you only have a short time limit to complete each section, if you go over, the tickets will be released), so having an account already makes the ordering process easier and quicker.
Last time I ordered over the phone was probably 10+ years ago, so I can't remember if they gave you an option to choose your seats. They might have. When ordering tickets in person, they did give an option to choose the seats, but I haven't done that in years either.