Buying a Wetsuit is a Complicated Choice. We have been Selling Wetsuits for 20 Years and there are some Fool Proof Rules you must Follow. Here are a few key tips when deciding how to purchase or rent a wetsuit.
1. Tight is Right. A wetsuit can not fit like a T-Shirt and Keep you Warm. Imagine a pair of baggy yoga pants and how those would not work for every pose. Loose wetsuits let water pour in and will not keep you properly protected. A wetsuit should be as form fitting as possible and may be slightly uncomfortably tight to stand around in while dry. 
2. Trying on the Suit and Getting an Evaluation by a Professional Diver is imperative. The Person looking you over is trying to find a way to make your experience better. Buying a suit sight unseen is not ideal.
3. Using the Correct Suit for the Activity / Region. One suit does not do all things for all People. In Southern California we are lucky to have adventurous and hearty divers that want to brave cooler water. But not all suits will carry you through the entire year. If you Plan to Dive only between the Months of August and December you may like a 5 Millimeter Wetsuit. But if You want Year Round Action, You are going to need that 7mm with Hood Attached or maybe a Hooded vest to layer up. Tropical Divers may just want a hooded vest over a lycra / poly skin to save weight on the flight and to dry quickly before boarding the flight home.
4. Free Diving Wetsuits are Not the Same as a Scuba Diving suit. If you have a nice Open Cel ( Think Sticky Soft Marshmallow ) Interior Freediving suit, Do not subject it to the constant pressures that scuba will involve. Also Freediving Suits are much more Difficult to get into and out of. Straps and buckles and additional weight of tanks may deform the delicate free diving suit or even tear it. 
5. How Much Do you Spend ? You Don't have to Spend a Fortune on a suit in these times. Every manufacturer has a suit aimed at your budget. Most 7mm Wetsuits low price are around $235 and go all the way up to $895 for Hood Attached, Horizontal Zipper Models. A 2mm Short Suit can be as Low as 49 Bucks.
6. Consider layering materials to keep warm. A Polypro, Lycra Skin, Hooded Vest Combined with a Heat Retentive Swim Suit or Shorts can go further in reducing buoyancy and keeping you warm than the most expensive suit can do in some cases
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7. Assess and Evaluate your own metabolism and comfort against cooler water and weather. We the Scuba Instructors of the world, are always teaching the dangers of Hypo and Hyper - Thermia. If you are the sort of person who is always warm, or maybe has a bit more Bioprene around the middle, try a thinner suit with a layer that you can add on the last dive of the day. If you feel a bit on the Cold Side then Get the best suit and layering garments you can afford and make sure they fit absolutely perfect.
8. Don't Forget to Cover your Head. Hoods Exist in Beanie Style 1 Millimeter, 3 Mil Short or Tuck In Bib Style all the way to 11 MM Custom Neoprene. again, we want to mention the hooded vest as a winner in least buoyant but most warmth champion. This type of garment is Sleeve less and can go over a 3mm Surfsuit to Create and instant and fairly efficient Freediving suit or it can be worn under a 5 or 7 mm for a cold water sustaining barrier.
9. Fashion - Oh So Many Colors and Materials to make you look like, well....Someone wearing a rubber suit. The BCD will cover much of the top half of the suit while diving and many times between dives its best to remove the suit and dress warm so don't let fashion sense over ride your common sense. The Fish really only like you for your inner beauty anyway. The more Gentle you are inside and the more amicable you are toward them is what they are drawn to.
Have fun and Stay Grateful.
The Team at HD.