So, it's a question of trade offs.
If you are looking to maximize a mechs capabilities, an XL or Light engine is the way to go.
Faster engines weigh more, so to get maximum speed you usually need an XL or Light engine.
More weaponry requires more tonnage, so you usually want to swap a Standard Engine for an XL or a Light, so you can fit more weapons.
If critical space becomes an issue more than tonnage, a Light Engine may be preferable to an XL. For example, if speed and tonnage are not an issue and you want to fit something like a Gauss rifle in a side torso, you might opt for a Standard or Light engine.
You may find that an XL engine makes your mech too vulnerable and easy to kill, in which case you may sacrifice some extra tonnage in favor of the survivability of a Light Engine or Standard Engine.
Light and XL engines are also very expensive, so you may opt to use Endo-Steel or Ferro-Fibrous to save tonnage, rather than incur the cost of a new engine (which is often nearly the same price as an entire new Mech)
As for sample builds,
I have a Black Knight and Supernova which use standard engines, which free up tonnage for weapons with endo-steel. Because of critical space limitations, mounting a different style engine is either impossible, or not worth the cost (minor improvement for extreme cost)
Most of my mechs use Light Engines. I generally am an IS Player. I often switch a Standard engine for a Light Engine of the same rating. Occasionally I can even fit a slightly faster engine. These let me upgrade to better weapons, but I can still survive the loss of a side torso and keep on fighting. So I can take a standard variant, upgrade to a faster light engine, add AMS, add armor, upgrade the weapons (like change a standard AC5 to a Rotary AC5), and now I basically have an improved, upgraded version of the stock variant.
I will have to look at my Mech Bay for an example of an IS XL build I use. One scenario where it makes sense is in a Light mech, where your survivability and available tonnage is already so low, that the extra weight makes more of a difference than the extra vulnerability. Or at the other extreme, an Assault mech. An XL engine could free up so much space that you can create the kind of ultimate firepower meta build that just erases other mechs off the map. And because Assaults have so much armor, the extra vulnerability of a large engine is a little less of a concern.
Edited by Lieutenant Hedgehog, 27 November 2020 - 09:05 AM.