So I know that there's a heck lotta debate between two factions: those who insist rushing HQ is better, and those who insist maxing HQ is better.
So I'm gonna list pros and cons of doing each.
|Rushing HQ||Maxing HQ|
Each point explained:
Unlocks more buildings, including more offense-related stuff which gives you better offense: Most of the time whenever you upgrade your HQ you'd get an extra Landing Craft, or say, new troop, or stuff like that. Extra Landing Crafts will have only advantages and no disadvantages. It would mean that your army expands relatively vastly (by 1/5th, 1/6th or 1/7th). New troops, they're worth it as you can use them in any situation. As a case study, consider a maxer at HQ13 and rusher at HQ15, and both are using a Tank-based army. Both would have a relatively similar XP level; yet the HQ15 has Medics while the HQ13 does not. Both can be expected to have level 12 Landing Crafts, so there's not much difference there. However, even though the HQ15 would probably have less Tanks used as some are swapped out for Medics it would make the HQ15's Tanks more viable against AoE defenses. Compared to the HQ13 whose only way to heal up the Tanks would be through a Medkit. In other words, the HQ13 is using all-Tanks which is pretty risky compared to the HQ15's Tanks and Medics.
No realistic penalty for getting all those buildings and having a rushed HQ and rushed defenses: This is somewhat easy to explain: There is no such thing as loot penalty among HQs in the game. So there's no problem in rushing the HQ.
Allows better levels for specific buildings compared to a similar-leveled player who maxes HQ instead of rushing: This is a good reason to rush the HQ. You get new levels for important buildings, say the Sculptor. Comparing a rushed HQ20 to a maxed HQ16: The HQ20 can have a level 8 Sculptor whereas the HQ16 can only have a level 6 Sculptor.
Another case study example: Say there's two players, struggling to defend against T-med so they want to max out their Boom Cannons for their HQ level. Say that both players are XP48; one is a maxed HQ17 and the other a rushed HQ20. The maxed HQ17 can only get their Boom Cannons up to level 9 whereas the rushed HQ20 can completely max out their Boom Cannons at level 14. In addition to that, the HQ20 can build 4 of those while the HQ17 can only build 3 which gives the HQ20 a much better chance at defending T-med than the HQ17.
Overall defense is more rushed and less effective; their standard might be that of a maxed HQ a few levels below the rushed HQ level: What would you prefer: 6 level 12 Sniper Towers or 5 level 15 Sniper Towers? Not only does the level 15 Sniper Tower do more damage it is less likely to be taken down easily by 2 or even 1 Artillery shell. Also; the fact that the damage done by level 12 Sniper Towers are easier to outheal by Medics than damage done by level 15 Sniper Towers. NOTE: This compares a rushed HQ17 versus a maxed HQ14, both which are approximately XP level 42.
Overwhelming upgrades; there's a lot to do: Well almost every building will have an upgrade to do, in fact they'll probably have at least a few (exceptions are likely to be Sculptor, Armory, Gunboat and Landing Crafts). Converting from a rusher to a maxer is a tough journey well because you have dozens of upgrades to do, quite possibly hundreds. For example, about 33% of building upgrades are done in a rushed HQ19 whereas even a maxed HQ16 would have approximately 42% of building upgrades done.
Makes sure all buildings are up to a good standard; no worries about rushed defenses: Well if you max your HQ then there's not a defense that would be rushed. As explained earlier, you'd have more of your building upgrades done compared to a rusher: as shown above a maxed HQ16 has approx. 42% done and a rushed HQ19 like mine, 33%.
Offense cannot always benefit as usually they are upgraded by only 1 level at a time; not always beneficial: Think about this in two different viewpoints: the Landing Crafts and the Armory. Not every one of the Landing Craft upgrades would benefit your T-med or Heavy-Zooka army, notable examples are levels 13, 15 and 19. Although they can buff up Riflemen armies it usually does not matter for other armies at all. As for the Armory, let's use the Heavy as an example: usually each HQ level above 4 will have one Armory upgrade and that means that Heavies will be upgraded once per Armory. Which means Heavies will usually be upgraded only once per HQ level.
Cannot upgrade specific buildings to high levels as compared to a similar-leveled player with a rushed HQ: This, in my opinion, is a significant con of maxing the HQ. You can't upgrade specific buildings like the Armory to a high-enough level; which means that you'll lag behind in offense and defense. Again consider the case study with the players who maxed out their Boom Cannons; 3 level 9 Boom Cannons will not be able to match up with 4 level 14 Boom Cannons.
Maxing each building is a harder job at a lower HQ level: Generally it is known that the higher your HQ level the easier it is to accumulate resources due to having a possibly better Vault and better offense. Consider upgrading all your Sniper Towers to 15 as a HQ14 compared to upgrading your Sniper Towers to the same level as a HQ20. It is clearly seen that it is much easier to upgrade the Sniper Tower as a HQ20 having possibly a better Vault and all (if its there).
In general, there seem to be better advantages than disadvantages to rushing the HQ compared to maxing the HQ. So I would definitely recommend rushing the HQ regardless of what position you are in. If you can go for the HQ upgrade, justly do so.
Again, you may always go ahead and max out your HQ, I will not criticize your decision. It's up to you in the end. Just hope that you won't regret it sometime in the future if you end up as a level 58 yet still having a HQ19 and losing out because you don't have the 8th LC.