For Sparks, they have a 50 second total rebuild time (10*5), which is quite long (and doubly so because sparks are not very tough!)
Sparks are toughest interceptors by quite a large margin.
Yes and no! In terms of per wing HP: 300 hull, 20 armor, 150 shield; total wing 'hp' ~2350, with low armor damage mitigation. 'hp/sec' regen from a deck: 47, but only if the deck stays at 100%, which as is the topic of this thread, sparks are terrible at thanks to their 50 second wing rebuild time.
Gladii: 500 hull, 75 armor; total wing 'hp' 1150 with much higher armor protection. 'hp/sec' regen from a deck: 57.5, and this is reduced much less thanks to only having a 20 second wing rebuild time.
How much of a difference does the armor makes depends on weapon. One of the more extreme examples is the vulcan (though lmg's will be worse): they have 6.25 damage for purposes of armor damage reduction, and 125 dps vs armor if all shots are hitting (which they won't but hey, lets just say they do to get numbers). The vulcan is pegged to minimum armor damage vs a Gladii until the armor hits 35, around 2.1 seconds. Damage starts to increase then and it takes less than a second to chew through the last 35 armor; call it .9 seconds as a guestimate for a total of 3 seconds. Spark armor takes about half a second for a vulcan to chew through (probably less, but lets round to .5).
In the HP calculations above the armor was counted 1:1, but as the above example shows for some weapons it can count as a lot more. The top 40 armor of the gladii is counting as ~1050 frag damage: its going to be a lot less vs heavier weapons. So... without a lot more detailed calculations its hard to know how much to 'weight' that armor, especially as it is so dependent on what the fighters are flying against!
In terms of hull damage, Sparks will have 1 residual armor vs 3.75 for Gladii. For again the vulcan, this makes the DPS of the weapon 431 vs Spark, 312 for Gladii. So .7 seconds for a Spark, 1.6 seconds for a Gladii. Shields is easy: 125 DPS vs 150 hp gives 1.2 seconds for the spark and 0 for the shieldless gladii.
In total, an estimate of a perfect accuracy vulcan (hah! doesn't exist) vs the fighters: ~2.4 sec/Spark, ~4.6 sec/Gladii. Total wing time: 12 sec for Sparks, 9.2 sec for Gladii. Damage vs repair ratio: 12/50=.24 for Sparks (discounting the quadratic effects which punish Sparks hard), 9.2/20 = .46 (higher is better).
So what does this actually mean? For this very narrow case of a low damage/shot frag weapon, a fresh Spark wing is about 30% tougher than a Gladii wing, but when it comes to pressure over time (IE extended battles, waves of fighters, etc) the Gladii is more than twice as tough as the Spark! The quadratic effects of wing replenishment are really hard to calculate as they depend strongly on real battle conditions and skills, but in the OP's case of the rate sitting at 30%, its quite possible that a Gladii would be
replenishing 'effective hp' 6 times faster. Again, the vulcan is a common weapon but many others are going to have vastly different numbers: kinetics will yield much better numbers for the gladii; HE will yield better numbers for the Spark; larger shot size frag will favor the Spark (as the Gladii will lose more of its armor effectiveness).
Other things ignored in the above: The Gladii flares and the impact that will have on helping other wings survive; the regenerating shield of the Spark for taking non-concentrated bits of chip damage; Sparks overloading from a big hit reducing their DPS to 0.
So.... yeah! Thats a whole wall of text because armor and fighter mechanics are complicated. But basically: Sparks have a good amount of up front HP. They have a poor HP recovery rate because their rebuild time and rebuild/wing is so long. Their overall toughness is not nearly as high as it looks 'on paper' vs more heavily armored fighters.
In terms of heavier fighters, Broadswords at 750/100, 10 sec rebuild, and Warthogs at 750/
200, 10 second rebuild, are way way tougher than both! They have their own downsides of course.