First, I want to wish everyone a Happy and Prosperous New Year!
Second, I recently got an email from an individual who wants to purchase a dSLR, but asked for my thoughts on whether a full frame or cropped sensor type camera should be chosen --especially since this would be the person's first dSLR. I have to say that I thought this was an excellent question and it really had me considering both the technical and personal usage sides of this query; in fact, it was the latter that I hadn't really pondered in the past.
From a technical standpoint, full frame dSLRs generally have the upper hand over their cropped brethren primarily in regard to producing higher quality images (e.g. less noise at higher sensitivity settings) and shallower depth of field versus cropped sensors (great if you're doing portraitures). In addition, many full frame cameras have higher-end features like weather sealing, more durable build quality and shutter mechanisms, along with faster sequential shooting amongst other things.
As I continued to discuss the topic in my email, I couldn't help but include the old adage of the law of diminishing returns. Full frame cameras are almost always more expensive (sometimes substantially) than smaller sensor cams and frankly, it's not like cropped sensor machines don't deliver excellent results (think images from the Canon 5D/7D, Nikon D7000 or D300s, or even the E-5 from Olympus, including even some entry-level dSLRs). Full frame dSLRs are also generally large and weighty, including their lenses, so this might not be attractive to shooters who like to travel light. But here's where the line started to blur and some new branches sprouted.
What are your intentions with the camera? How are you going to use it? Are you just an enthusiast photog and will mostly record precious personal memories with the device with some artsy work thrown in? Are you planning to create large prints from your work and sell them? Or perhaps you want to become a professional photographer who does weddings or shooting for corporate clients?
Answering these questions, as well as others that may suit the scenario, will likely ease the decision that needs to be made. For interest's sake I've added some of my reasoning to a couple of situations, which might help you decide if you're ever faced with such a quandary.
If you're a hobbyist photographer, a full frame camera will likely be overkill, unless you have amassed some impressive personal wealth and spending upwards of five figures is pocket change so you really don't care. But otherwise, such a system will be quite expensive, many features will probably go unused and on the money saved buying a cropped type camera, one could invest into buying a few good lenses instead. Like I wrote in my email, I'd rather have a larger selection of lenses than a slightly better camera body with fewer pieces of glass. And if you plan to do some part-time shooting for money, say next to a day job like yours truly, even that may not justify getting a full frame beast assuming there are no special requirements (e.g. huge prints, like poster size or larger; plus see below). Even as a full-time photographer you may be able to employ a cropped sensor camera for things like weddings, portraitures and various other corporate work, and have little to no issues at all with such a camera type.
But what applications could a full frame camera be used for where you would really see it shine? Well, quite a few places actually. To begin with, most full framed machines have more pixels, thus work requiring larger prints will demonstrate their superiority in terms of resolution and cleaner images (less noise). Many can also shoot around eight to ten frames per second versus their lower-end buddies, so sports/action photographers will appreciate that. And many such photogs are also in environments where unpredictable weather could be an issue. Although not in all cases, most full frame models are sealed so they can handle higher levels of humidity, including being in rain or snowfall (but it's worth mentioning that there are several cropped sensor cams which are weather sealed too). Shooting in places where there's a lack of light or flash is prohibited for whatever reason makes full framers stand out. Since the sensors are quite large, the photosite (aka pixel) density is often lower, thus using higher sensitivity settings will usually result in images that are more usable compared to noisier cropped sensor sized cameras; serious astrophotographers might also see a benefit in this arena. Lastly, portrait photographers will achieve shallower depth of field more easily with such equipment, whereas cropped sensor users may need to find potentially more expensive fast lenses to get comparable results. I'm sure there are even more reasons, but I'm sure you get the idea.
Obviously there are far too many factors (personal, technical, financial, etc.) for me to go through, so it goes without saying that you'll ultimately be the one deciding on what to go with, if you're in such a position. Nonetheless, I found it quite interesting to consider such a proposition and mull over how I would handle this, and I certainly hope you find this useful.