Are aluminum or steel chainrings more resistant to bending? I mash a lot and wind up killing chainrings. I later find that my chainring warps at the midpoint between two chainring bolts. Usually my first sign that something is wrong is my chain pops off or breaks and I can see the bump in the chainring that caused it.

I've gone through a 3 or 4 34t/36t chainrings between my single speed track bike and geared cross bike bike with a shimano deore drivetrain. Both have bash guards protecting the outer ring, so I doubt impact is a factor. 

When replacing these, what is going to resist bending more, steel or aluminium?
As far as My experience gets on mountain biking, cheap steel is more resistant than cheap aluminum, with expensive parts the story changes a lot because many different alloys are used by different manufacturers or even the same manufacturer changes the formula among models.

The advantage of steel anyway is that it can be safely unbent as long as the deformation is not too dramatic.

However, in my case, most of the bent chainrings come from hitting objects on rough trails, except one time that it was due to one of the chainring bolts comming off. I have been kind of a masher myself, but I have never bent a chainring due to pedaling force (I have broken a couple cheap freewheels due to this instead). So I think some other factor may be affecting your problem. A couple of them are:


Chainline: The pull on the chain should be pretty straight, so even under extreme forces it is unlikely to bend a chainring/cog. However a bad chainline makes the chain pull in a direction that is too unalligned with the chanring's plane. This can be due to a bad combination of components, specially a bottom bracket of the wrong spindle length, or, more commonly, a bad choice of gears. In a mountain bike the innermost chainring should be used with the 3-5 closest-to-hub cogs, the outermost chainring should be used with the 3-5 furtest-from-hub cogs and if there is a middle chainring it shoud be used (you guessed it) with the cogs in the middle of the cassete. Check the chailine of the gearing you are using most by looking at the chain so your line of sight is aligned with the point where thr chain leaves the cog and with the point where it enters the chainring. If the chainline is wrong you should esily note how the chain bends at the entering-the-chainring point.
Frame flex: If your frame flexes easily and you are mashing your pedals, both factors add up so when you are applying maximum force, components are most unaligned, causing forces to actuate on parts at wrong angles, thus making them prone to deformation, but the problem is not evident while evaluating the bike onthe workstand.
Bad shifting technique: Shifting gears while applying too much force causes all kind of transmission problems, from bent chainring teeth to bent chain links, broken chains, premature wear and even derailleur malfunction/damage. Proper technique, specially for downshifting while masshing pedals up a steep slope is to a) plan prior to shifting, specially shifting before you exhaust your momentum. b) Accelerate a little just before the shift. c) Ease the force on the pedals while at the same time applying the gearshift. During the whole shifting process you should practically just spin the cranks, you rely on innertia to continue in movement and that is what the previous acceleration was for. d) resume normal pedaling once the shift has completed and the transmission is working silently again. (noise is often a sign of bad shifting).
Bent parts: There may be something else bent on the bike. Particularily the crank spider or even the whole crankarm. If thats the case, it is making you loose the chainline at least two times per pedal revolution, once towards one side, once towards the other. This has happened to me with cheap, soft aluminum, square taper crankarms: with heavy use, the spindle interface deformed, causing the whole crankset to rotate off axis causing a set of not-so-funny symptoms, including chainsnap and front shifting difficulties. Careful inspection on the workbench wile manually rotating the cranks is advised. If this is the case, it is diagnosable by eye or a simple ruler/metric tape. A bent frame is less likely but not impossible, however, it may be harder to diagnose.