Hi Anthonydh,
You should certainly book whichever time slot would make you most comfortable on Test Day (re: based on your practice work so far, at what time of day do you typically perform at your best?). Since the Test Center employees can only 'start' one Exam at a time, it's likely that all of the people in the Computer Lab with you will have different 'start times' (whether it's 1 minute, 15 minutes or some other length of time). Thus, there will be people entering and exiting the Computer Lab throughout your Exam (and you'll be entering and exiting too). There's no way around that particular issue - but most Test Facilities offer some type of noise-blocking option (ear plugs or headphones.
Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
1) When are you planning to take the GMAT?
2) How long have you studied?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores for each)?
4) What is your goal score?
GMAT assassins aren't born, they're made,
Rich