Since the discussion has shifted around a lot, I’ll do my best to isolate the exact differences between the two gems mentioned in the original post.
While the bonus to impaired enemies does indeed start at a 15% bonus, it grows a lot. At rank 100 this bonus is 45% and maxes out at 60% with a rank 150 gem. This damage bonus is multiplicative to any damage you do from any source. The only caveat is that you need to slow/chill targets consistently.
At gem rank rank 25+ it provides a small 15 yard chilling radius (like a personal air conditioner), but this won’t help ranged builds much. Thus, items like Pinto’s Pride or skills like Thrill of the Hunt are ideal with this gem. In short, for most common builds, you should have this bonus applied for the majority of the damage you deal.
This gem simply adds a poison damage-over-time debuff to any damage you do. The 10 second duration is refreshed when you hit a target again (it does not stack). Next, the poison damage it deals is not multiplied by your skill damage or set bonus (unless the set bonus boosts all sources of damage, e.g. LoN). Thus, for many builds, the poison damage is negligible.
At gem rank 25+ it increases the damage monsters take and reduces damage they deal by 10%. This 10% bonus damage is additive with many other multipliers: skill damage increased by 10-15% (found on class-specific items), Strongarm Bracers, Marked for Death (skill), and many others.
You now need to compare: doing +15-60% more damage to anything chilled or slowed as a multiplicative bonus, to +10% more damage to anything poisoned as an additive bonus (the real bonus might be significantly less depending on the build). Thus, the Bane of the Trapped is the superior damage gem in almost all situations.
That said, the Gem of Efficacious Toxin does have a use in that it applies the damage buff for all players. Thus, a supporting-role player can use this gem to help the party deal more damage and take less damage.