<p>After I updated the firmware on my Prusa i3, the Bed won't switch off anymore. It worked perfectly before the update, but now, the moment I power up my printer, the LED on the bed turns on and it starts heating up. The manual control in Repetier Host doesn't turn if off or on and I even tried g-codes <code>M140 S0</code> as well as <code>M0</code>, but it does not switch it off.</p>

<p><strong>Edit:</strong>
I have an Arduino Mega2560 with a RAMPS shield. The Marlin firmware came pre-configured from the store I bought the kit from.</p>

50°C is hot for you. PLA's glass transition temperature is 65°C. A car in the mid-day sun can get very hot indeed.
As to the question of "stronger" - if the part is designed to be
strong enough
for its use in PLA, it will be no better in "stronger" ABS. If the PLA part will be "more precise" and "less warped" - that may well make it better for its use. Other than a widespread community dedication to self-replication, (or mostly self-replicating with some metal parts) there's plenty of arguments for making most printer parts out of machined metal, for that matter - much stronger than ABS or PLA.
Brittleness likewise might matter if you were planning to drop the printer; but should not matter in use if the part is designed for the forces it will see in the material it's designed for. Looking at older technology, many machine tools are made of cast iron, which is quite brittle, rather than steel, which can be less brittle depending on how it's processed. If the part is designed for the job and used/handled in a reasonable way, "brittle" is not an inherently bad thing, and a thin steel part that flexes is a worse thing than a thick cast iron part that flexes less.
As for
withstand the elements
that appears to be a reference to outdoor use where PLA will break down - but if you are not leaving your printer out in the rain (which will be bad for many other parts unless you do a lot of engineering to protect them) that should be of little concern.