One of the big buying decisions photographers make these days is whether to purchase a full-frame or cropped-frame sensor camera. Back in the day, a 35mm SLR was a 35mm SLR.
Today, a small format, 35mm dSLR is one of two things: It either matches the frame dimensions of analog 35mm SLRs (full-frame) or it records a smaller portion of the traditional 35mm frame via a cropped-frame sensor. (APS-C size sensor.)
Often, the decisions of which to buy is decided by money: Full-frame dSLRs cost more than cropped-frame dSLRs. (D'uh.)
Some photographers, depending on what they mostly shoot, let their preferred genres be their big deciding factor. Someone who shoots a lot of nature (flora and fauna) might choose to go with a cropped-frame camera. Someone else who shoots a lot of nature (landscapes) might choose to go with a full-frame camera.
Generally, the kind of glass you mostly use helps make the decision. If you're most interested in shooting the flora and fauna side of nature, you might opt for a cropped-frame because you want to extend the reach of your long lenses. (You don't actually extend their reach, i.e., lengthen the focal length of telephoto lenses, but it seems like you do.) If you're most interested in photographing the landscape side of nature, you might opt for a full-frame because a full-frame lets you take full and best advantage of your wide angle glass.
With people photography, the decision to go with full or cropped-frame becomes a little less obvious. (Money aside.) For shooting glamour, I much prefer my full-frame dSLR. (A Canon 5D.) Since I shoot most of my glamour with a telephoto lens, i.e., anywhere from 85mm to 200mm as my focal lengths, using both prime and zoom lenses, I want to be able to utilize the full frame.
Even though I might be shooting full-body shots with a long lens, the 5D's full-frame means I don't have to back up as far in order to frame the model, head-to-toes, in my viewfinder. That's not to say, of course, that terrific glamour photography can't be produced with a cropped-frame dSLR. It can and often is! I'm mostly talking about convenience and efficiency.
For most portraits and headshots I don't care as much if I'm using a full-frame or cropped-frame camera. When I'm shooting headshots, for instance, using a long-ish telephoto lens, I'm only framing the subject's head and some portion of their upper torso. The frame-size characteristic of the camera, in this case, becomes less important.
There are other factors when considering full-frame versus cropped-frame cameras.
Full-frame cameras generally produce less noise than their cropped-frame kin. That's because camera-makers use smaller pixels on their smaller sensors. Full-frame sensors also record greater dynamic range. They "see" into shadows better and also have greater abilities to handle the details in highlights. (Dynamic range is also a product of pixel size.)
In a nutshell, what all this noise and dynamic range and pixel stuff means is that full-frame sensors will capture images with greater quality and resolution. This becomes more and more obvious as you enlarge images captured with either full-frame or cropped-frame cameras. The images from a full-frame camera will noticeably trump those from APS-C sensors as you make the images bigger and bigger.
I talk a lot more about cameras and glass and other gear in both my ebooks, Guerrilla Glamour and Guerrilla Headshots. I also talk a lot more about stuff that's even more important; more important than gear, that is.
The pretty girl at the top, the one demonstrating her butt-flossing technique, is Penthouse Pet Tori Black. I snapped Tori with my Canon 5D and an 85mm prime lens. Had I been using a cropped-frame camera, it would have been difficult to get as much of Tori into the frame (as shown) because, when I shot this, my back was nearly to a wall in the room where I was photographing her.
Tori captured at ISO 100, f/5.6 @ 125. Three lights plus lots of ambient employed: Main light modified with my 5' Photoflex Octodome, a couple of kickers, way behind her on either side, modified with small, shoot-thru umbrellas. There was lots of big windows in the room plus a large, overhead, clear-glass skylight so I was able to take advantage of all that ambient.