XL vs. STD
One thing to note about engines: Regardless of their type, if an engine has a rating of 200, it will give the same amount of power regardless of type. Your mech will move and turn just as fast with an XL 200, as it does with a STD 200.
Now, on the engine explanation:
Standard Engines (STD) are available in every mech, they are fitted only in the center torso and as such don't take up any space elsewhere on the mech. That has the added advantage of allowing the mech to soldier on even after losing both side torsos. As such, they are very common in Brawling mechs (mechs built for close range combat and remaining in the fight or extended periods of time). Not to mention brawlers need to learn how to soak up a LOT of damage, and STD engines allow them to last for exceptional periods of time while under fire.
Zombie mechs are a special breed of brawling mechs (yes there are long range versions of them, but zombies were developed mainly for close range combat). They are designed around the concept of losing both arms, and both side torsos while still fighting. The Centurion chassis is one of the most famous/infamous zombie mechs in the game. To give you an idea, a Zombie centurion can continue fighting even while at 8% percent hull integrity (no arms, no side toros, one leg, everything is cored yellow and red, but it still has 2 MLs in the CT).
Such survival potential is not possible with an XL engine.
As such, STD engines are ideal for mechs that fill these roles/requirements. The trade off is that they are heavier than XL engines.
Extra Light Engines (XL) are an upgrade with a trade off: They are lighter so your mech can have bigger ratings for the same weight. For example a STD 220 weighs 14 tons, which is the same weight as an XL 275. That's a 3 Heatsink difference (8 heatsinks in the 220 Vs. 10 in the 275 + 1 internal) plus a staggering speed increase.
The downside is that they occupy 6 slots in the CT (same as a STD engine) plus 3 in each ST (Side torso) MW:O rules state that an engine is completely destroyed if it loses 3 or more slots, as such, losing a side torso is instant death.
XL engines do have a risk with them, however they are designed for a different role than STD engines. While STD engines focus on survival. XL engines allow you to put bigger engines into your mech, as such you would be more maneuverable and more mobile.
XL engines are used in mechs that need to get around the field quickly, while carrying a lot of things (Dakka falls under the all benevolent category of "things") As such, fire support units such as LRM boats, and sniper mechs, or Skirmisher and harasser mechs such as small LRM load mechs, and 2 LL toting mediums and lights engaging at over 500 meters, could definitely use XL engines. Mechs in those roles are not meant to deal with a lot of firepower directed at them, as such they can afford to mount XL engines to allow them to move faster than with a STD engine. (STD 160 weights 7 tons, while an XL 190 weights 6, the 190 allows you to move more than 15 KpH faster, which is a significant difference) On the other hand, those roles are required to move and relocate quickly all over the map to provide support for their team. So the speed is more valuable for them than survival.
Light mechs are an anomaly to this rule, as whether they are brawlers or scouts, or skirmishers, they should be mounting XL engines more than 90% of the time. since they need to save every ton possible, and faster movement can mean the difference between life and death.
Special note: The three slots that an XL engine takes in the STs are not critical slots in the traditional sense. Apparently they do not get hit by critical strikes. The only way to really destroy those 3 slots is to destroy the entire side torso.
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