I bought a RCBS single stage full O frame press back around 1983 or 1984 that is made of aluminum, and have yet to wear it out. At one point I thought it had worn out, but after completely taking it apart I discovered a piece of debrie had wedged between the ram and the frame, probably from a spent primer. It was causing the ram to drag and and hang up and had been going on for some time, so I thought the press had worn out. Despite taking the press apart more than once and cleaning it, I didn't see the problem. I'm fairly certain that wouldn't have happened with a steel frame, but that has been the only problem after 25 or so years. My steel O frame RCBS press is no more rigid than the smaller lighter aluminum one. In fact, neither press has performed undesireably other than the one mis-hap. My brother in law bought a Lee 25 + years ago that has a connecting link, rather than the full O frame design, and it isn't as rigid as a O frame type, I don't personally care for it. I can actually see it flex when sizing bottle neck cartridges.
I use both presses just because of the convienence of setting one up for resizing with the other for seating when I'm loading bottle neck brass. The steel press has the priming arm, but I've never used that feature as Iuse a priming die.
If you plan on reloading a good deal, it would be my opinion to buy a good O frame press, and RCBS makes a high quality press that will likely last a life time. I can't advise you of what they cost these days, but I'm sure it is worth it to get a good solid O design, rather than a less expensive non O design. Other than the frame on the aluminum RS-2 press, all of the other components are made of solid steel.