What you are seeing is different body types of bettas.
The 'droopy' ones are probably veil tails- the original fancy bettas.
The spiny ones are crown tails (my personal favorites)
Spade tail has a tail that comes together at a point
Halfmoons are the current "this is the ideal, breed for this", and especially hard to get, which is why they are expensive.
A true halfmoon should have a tail that makes a vertical line close to the body when they display. They generally have the fullest fins.
You can also mix things like crown tail and halfmoon. A delta tail is when you're trying for halfmoon, but that leading edge isn't completely vertical, but only spreads to a slant. These are often sold as halfmoons in stores, so be careful what you're spending your money on.
Double tail (and I've heard of triple and heart tails too) - this is a normal growth pattern, not damage or illness.
A newer type I've seen on the market are the Dragonscales. These guys have a high, metallic sheen on their scales, and usually have it up to the head, where many other bettas have black over the front half of their body. All of the dragonscales I've seen have had the plakat fin type, very short, natural, wild fin size.
Plakat - don't confuse these guys for females!
http://watershed3.tripod.com/types.html here's a nice page with decent pictures of many tail types and colors.
Some bettas will have extra ballooning (finnage) between the rays (the supportive spines). Some folks like me, think it's very pretty and encourage it. Some folks don't like it and avoid it. Sometimes, the bettas get blown around their tank too much and may even chew their own fins off if they have too much ballooning, which would be a reason to avoid it. Sometimes, the rays in the fins get curves or curls. Again, some people like the effect, others don't. i actually had difficulty finding any good photos of bettas with healthy, curled fins, most of the photos are of straight fins. But, here's a crown tail with ballooning and a bit of curl, and I found another with lots of curly frills.
Here's a betta with fin rot. Look for fish with even fin lengths, not ragged like these
Also, avoid clamped fins
Rolled up fins is another thing to avoid. It's an illness that usually responds well to fungal treatments, but I'm not certain exactly what causes it.
Fins will grow back, so if there is a split or a chunk missing from the finnage, but the fish is otherwise active and appears to have a healthy body, and the color is one you like, go ahead and go for it.
Pretty much look for the same things in any fish. You don't want to see them laying listlessly on the bottom of their cup. Even when they are resting, they should stay vertical. You don't want to see them in a cup full of old, decaying food. Don't get any with any kind of white spots or patches (that aren't normal coloration)This is not a sick betta. This is normal white patches of color.
The scales should all be smooth, not sticking up.
Even pastel or white bettas should look bright, vibrant and glossy in their color. Sick bettas will sometimes loose their color and just look pale, even white, like this poor guy.
I, myself, just bought a female betta who I thought was a neat yellow and orange color. It died that same night and I realized that she was supposed to be a green and red, and had just paled out. She had been sitting well in her water and appeared active, I didn't realize she was sick. There were two others in cups on the same shelf who looked like her, all appeared active with no other signs of illness. I figured it was just a color morph. When I took her body back for the guarantee, I noticed the other two were gone as well. They probably also died.
http://splendidbettas.webs.com/commondiseases.htm here's a great page for helping diagnose fish diseases. Not all of the photos are of bettas, but it's a great resource.
Anyways... I would do some online research, get really familiar with what is normal for betta build and color, so that you know when the fish you are considering looks healthy, and when you need to avoid it because something about it just looks 'off'.