Kyle,
In all honesty, the less you rent the better off you are. No matter how reputable a shop or a charter boat is, things can indeed fall through the cracks. Can you be 100% sure, absolutely positive enough that someone hasn't made a mistake with a peice of gear that can't be life-threatening to either yourself or a loved-one?
The smart choice is the start with the life support equipement. The computer choice is very important as it makes the biggest impakt on how you dive. The user-friendliness and feature's of the computer are very important. It'll not make mistakes, not lie, nor forget and be very honest with you. If you do something wrong, it won't be shy, it'll let you know you screwed up. A computer will accurately track multilevel diving giving you more bottom time because it gives you credit for fractions of times spent through the water column. It'll know what the water temp is and whether you have been breathing harder than normal, and adjust your No-Deco limit according to your new resperation rate. Let's face it Kyle, computer's are much smarter than we are...
The regulator's make a huge impact in your comfort level. Even in recreational depths an easy breathing regulator with a nice flexible hose, comfortable mouthpeice and a diver adjustable valve is key in diving comfortably.
The way a B.C. fit's properly and supports the hardware that it's supposed to can't be ignored. Less impakt on your back and your hips means less pain after the dive. More attachment points means more accessory options that will allow you to take more things such as safety devices with you. The package won't be intrusive and will add to your level of comfort because you know it's there.
Thermal protection. A proper fitting wetsuit, will insulate you many times more than one that doesn't. Knowing that your inner thigh(amongst other things) isn't rubbing up against a part of the suit someone Pee'd in yesterday... or it could be worse, but we won't go there.
The right size and type of cylinder that's right for you. Often times you can't rent a high quality cylinder from a shop. Most of the time they only offer Aluminum(yuck!) A good steel cylinder will have better buoyancy and balance characteristics, be in a smaller & lighter package than a comparable Al cylinder... how could you go wrong? If you had the option of taking more air with you, have it be in a smaller package and therefore more hydrodynamic, have it weigh about 5lbs less than what you lugged around before, and at the same time it allowed you to put less weight in your B.C. or weight belt... wouldn't you do it? Wouldn't anyone?
This could go on and on... it's one of my strong points as I have a background in Teaching, retail store management and the manufacturing end of the industry. Let me know if you have any specific gear or brand related questions and I'll help you out... till then,
Sea you in the Deep Blue,
Mario