A quick comparison of space weapons' [CrtH] and [CrtD] (disregarding [Acc] and [Dmg], for the moment...):
Take 100 hits with a weapon, that deals 1000 damage per hit. (
Simplified for ease of math. 
)
Total Damage Dealt: 100,000 Dmg
Factor in standards for Critical Chance and Critical Severity:
Critical Chance: 2.5% = total of 2.5 Crits per 100 hits
Critical Severity: +50% Damage per Critical hit scored
Standard Critical: +500 Dmg per Critical
Thus, for example used: +2.5 Crits (still count as normal hits, with an extra +500 damage per Crit, in this case, (2.5 * 500) = +1,250 Dmg
Adjusted Total Damage: 101,250 Dmg
[CrtD]:
Add in a [CrtD] modifier for +20% Damage per Critical hit Scored
Critical Severity: Now +70% per Critical = +700 Dmg per Critical
(2.5 * 700) = +1,750 Dmg (a difference of +500 above standard, approximately the value of one, additional Critical hit)
Adjusted Total (with [CrtD]): 101,750 Dmg
Adjusted Per-Hit value of [CrtD]: +500 Dmg (above standard) / 100 Hits = +5 Dmg per Hit
[CrtH]:
Add in a [CrtH] modifier for +2% Critical chance
Critical Chance: Now +4.5% (4.5 Criticals per 100 hits)
(4.5 * 500) = +2.250 Dmg (a difference of +1,000 above standard)
Adjusted Total (with [CrtH]): 102,250 Dmg
Adjusted Per-Hit value of [CrtH]: +1,000 Dmg (above standard) / 100 Hits = +10 Dmg per Hit
Now, this is simply a straight comparison between the two, factoring [Acc] into the mix can provide some balance for [CrtD], as excess Accuracy, when used against a Target's Defense (excess, in this case, being the amount by which your Accuracy roll beats their Defense score, in the engine, behind the scenes), is added to your Critical Severity (which is why Tractor Beams or Beam: Target Engines skills are critical to the 'one-shot wonder' kills).
These traits are
only factored for the individual weapon with that trait, so, if you have two Beam Arrays with [CrtH], each of them, individually, will have a +2% to their Critical chance, they will
not gain a 'cumulative' +4% chance. Captain traits (mainly Accurate), however, are applied to all of your weapons, thus, if your Captain is Accurate, all of your weapons gain the +10% Accuracy bonus, at all times.
In game, the differences between the two modifiers are negligible, but the math generally supports that the more often your weapon fires, the more valuable [CrtH] becomes. Conversely, slow-firing weapons (torpedoes and Heavy Torpedoes, in particular) tend to benefit much more from the spike damage that [CrtD] provides, but, it really comes down to player preference.
One aside, any user of Beam: Overload (any version), should strongly consider getting as many [Acc] modifiers as possible for the weapons that they employ for the skill, as there is nothing more frustrating than missing with a spike skill that costs you -50 Weapon Power.
Hope that helps,
-Big Red
Oh, and MustrumRidcully, my friend, Antiproton weapons have a built-in [CrtD] as their 'special', not [CrtH]... Definitely wish it was the other way around, though. 