Disclaimer: I was only an hourly for two months, and I was a sorter during that time. The answer below applies to working in the Hub, and does not apply to Preload operations.
I've been running new hire orientation in my building every week for the last three years, so I get this question a lot. I've trained new hires in both jobs, which often requires me to handle a large chunk of the work in order to both show the employee how an experienced person does the job, and to ensure that the new employee isn't overwhelmed. Keep in mind that I'm getting employees during their first 4 days on the job, so by the time I pass them along to their next trainer, they should be ready to handle more work without assistance.
From being around new hires pretty much all the time, this is what I've observed about the differences between unloading and loading:
Unloaders get fewer hours. If money is a major concern for you, the extra $40-50 you pick up weekly as a loader might be enough to make loading the better choice as a new employee.
Unloaders are expected to move MUCH faster than loaders. In my building, 400 packages per hour is a typical flow rate for a loader, while 1200 PPH is a typical rate for unloaders. Unloaders are essentially expected to handle three packages in the time it takes a loader to handle one. A "sprinter" will prefer the unload, while a "marathoner" will thrive as a loader.
Loading requires more thought than unloading (one of the reasons why loaders don't move as quickly). It takes more thought and effort to build a wall safely and properly than to tear down a wall that has already been built. Loaders also have more essential tasks to learn, such as breaking jams in their chutes, sorting and moving irregs to their proper trailers, recycle/re-entry, or entering information into their scanners. I find that I can train an unloader to work independently in about 4 hours of actual hands-on training. Loaders take about 6-8 hours to get to the same point. I schedule 1.5 to 2.5 hours of hands-on training per night during orientation.
If it were up to me, I'd start loaders at a higher pay rate than unloaders to better reflect the initial stress and learning curve associated with the job. Once you learn the job, however, loading isn't really that much harder than unloading.
If you're looking to move up with the company, regardless of which route you take, you'll definitely want to be competent in the loading aspect of the operation. This is especially true if you're looking to go into management for some reason
In my building, there are 30 PT supervisors on my sort, and only 8 of them work in primary (Unload, Sort, Irregs) capacities. In addition, there are 3 FT supervisors in the outbounds (where the loaders work), compared to 1 in the Primary. Not that this should mean anything to you, as no one in their right mind would ever agree to do that job! I'm sticking with the training department until I can learn more about/decide between IE and Simo for my full time move
TL;DR: If you want your job to be easy EARLY ON, be an unloader.
If you don't care about the above, loading is better for you IN THE LONG RUN, both financially and in terms of operational knowledge, should you need that experience later in your career.