Gladly.
First off, i need to clarify how the two work, since they work substantially different to one another:
LOL boosts come in two variants, but the net effect is the same: XP income per time unit played is doubled. You can spend more money to have them apply to more time units played, but you can't increase the effects of the boosts. They do nothing else. (I assume rising in level also pushes one into higher level matches against stronger players.)
Meanwhile CoC gems do a multitude of things: They permit access to items that are otherwise inaccessible and increase competitiveness. They are very powerful multiplicators in allowing constructions times (that i assume to span days in the extreme) to be entirely removed. Lastly, they allow the player to ... play more by restoring heroes.
So, the practical effects are:
LOL XP boosts do not increase competetiveness of the player. In fact, they can detrimentally affect it by pushing a player into high level matches before they themselves earned the necessary experience to function well in them. Further, players are discouraged actively from spending too much on them, since they may be stockpiled, but not increased in power and past a certain amount of stockpiling it's senseless to buy more.
CoC gems increase competitiveness fo players massively, in some cases (build times) applying literally infinitely large multipliers compared to the x2 multiplied of LoL xp. Further they also serve lock the player out of playing the game unless the player spends real money to buy and spend them on healing their hero.
TL;DR:
LoL xp boosts are fixed subscription fees with a maximum cap.
CoC gems are literally consumable cheat codes designed to encourage unlimited spending.