TL;DR: Subscribers drain field experience about 10% faster than F2P + EXP Booster, all other things being equal.

I recently had a chance to compare field experience drain rates for a Free-to-Play character using the Experience Absorption Booster from the Simucoin store versus a standard subscription character. These characters both had the exact same mental stats (40 Discipline/Intelligence/Wisdom) and the comparison used skills that were as close as possible in rank within the same skillset (Primary/Secondary/Tertiary) and also on the same wall rank cycle (all even numbers). Neither character was under the effect of any RP Awards, EXP Bonus Pools, Appraise Focus, etc. Although to my knowledge it's not relevant to the comparison, for reference purposes the F2P character was a Human Warrior Mage and the subscribed character was an Elf Ranger.

Accounting for the slight difference in some of the ranks, there would appear to be about a 10% difference in drain rates in favor of the subscriber. This seems to be what the store refers to when it describes the Experience Absorption Booster as being "almost as fast" as a subscriber.

Here is the data I collected:

SKILLSET	ACCOUNT TYPE	SKILL	RANK	PULSE
Primary	F2P EXP Boost	Augmentation	300 00.04%
Primary	F2P EXP Boost	Augmentation	300 01.31%	+1.27
Primary	F2P EXP Boost	Augmentation	300 02.59%	+1.28
Primary	F2P EXP Boost	Augmentation	300 03.86%	+1.27
Primary	Subscriber	Locksmithing	326 65.77%
Primary	Subscriber	Locksmithing	326 67.09%	+1.32
Primary	Subscriber	Locksmithing	326 68.40%	+1.31
Primary	Subscriber	Locksmithing	326 69.71%	+1.31
Secondary	F2P EXP Boost	Melee Mastery	234 92.40%
Secondary	F2P EXP Boost	Melee Mastery	234 93.50%	+1.10
Secondary	F2P EXP Boost	Melee Mastery	234 94.60%	+1.10
Secondary	Subscriber	Heavy Thrown	242 75.56%
Secondary	Subscriber	Heavy Thrown	242 76.74%	+1.18
Secondary	Subscriber	Heavy Thrown	242 77.92%	+1.18
Tertiary	F2P EXP Boost	Perception	206 46.11%
Tertiary	F2P EXP Boost	Perception	206 46.87%	+0.76
Tertiary	F2P EXP Boost	Perception	206 47.63%	+0.76
Tertiary	F2P EXP Boost	Perception	206 48.38%	+0.75
Tertiary	Subscriber	Performance	216 10.29%
Tertiary	Subscriber	Performance	216 11.12%	+0.83
Tertiary	Subscriber	Performance	216 11.95%	+0.83
Tertiary	Subscriber	Performance	216 12.78%	+0.83

Interesting, do you recall what mentals/ranks that was with?