The dDex portion caps at 15%; the overall crit rate caps at 100%. Without atma, you're only ever likely to cap crit rate with Rogue's Roll at ~3000 TP, which is an unrealistic usage scenario. +crit rate in gear is virtually always going to be useful.
Regarding the remainder of the comparison, it doesn't require spreadsheets, it just requires that you understand the basic math that goes into calculating damage. It also doesn't require 'perfect' scenarios for DPS calculations, because you're not calculating DPS; you're just calculating one component -- weaponskill damage -- and it's very easy to manage the realistic boundaries of the values that affect that.
Pillager+1 vs Plunderer+1 is:
2% crit damage, 20 acc
vs
7 str (4-5 att, 1.75 fStr), 2 dex (1 base damage), 4% crit rate
Assuming dDex was already capped with Pillager+1 (to maximize the value of the crit damage, and negate the dDex advantage Plunderer+1 has), and that you had something like +10% crit rate and +9% crit damage in gear, plus the base weaponskill crit rate, the remaining 2% crit damage you get from Pillager+1 works out to about +1.0% weaponskill damage, up to about +1.2% at 1500 TP (maybe up to +1.3% at low cRatios).
2-3 base damage from WSC and fStr with Plunderer+1 should be somewhere in the range of +0.75% to +1.2% weaponskill damage (assuming total base damage of around 260). However, worst case, you don't get anything from fStr; so, minimum is roughly +0.40% weaponskill damage solely from WSC. 4 att (if not capped) is also about another +0.25% damage (after accounting for crit rate and different relative gains), though that might also be capped. And 4% crit rate is worth around 2.5% to 3% damage, depending on cRatio (lower value as you approach att cap).
Overall, Plunderer+1 is worth between +2.9% and +4.5% weaponskill damage compared to the neutral baseline, vs the (best case) +1.3% from Pillager+1. Also, add a small bit more to the Plunderer+1 value if Ambush comes into play.
Really quite clear which will give higher averages, though of course if acc matters, Pillager+1 wins easily.
Also, if you're comparing Pillager+1 to Plunderer NQ (since the HQ is far more expensive to make than the Pillager+1), Pillager+1 wins (should be obvious), but in NQ vs NQ Plunderer wins again.