There are a ton of factors determining strength gain kept, such as diet, training methods etc. But here are some of the most important underlying factors....
1. How close are you to maximum motor unit activation with an optimal discharge frequency (CNS system, absolute strength, explosive strength)
2. How much of your strength gains were attributed to the increase of the cross sectional diameter of the muscle fibers or increased muscle fibers (Hypertrophy)
(It has been shown that strength gains due to hypertrophy decline slower once training stops than CNS related strength gains, so I'm sure as long as you can keep the size they are easier to maintain too)
3. How much the steroid manipulated the cns system or temporarily increased the size of the muscle cell with water (You do get stronger with bloated muscle, it swells the muscle in size, therefore changing the leveraging angle of the muscle in a positive manner)
So generally speaking in the physical aspect of things, you'll keep a higher percentage of strength gains from a compound that does not get the majority of it's gains through temporary water or cns manipulation, but through a true increase in hypertrophy.
I have taken small dosages of anadrol and had great strength gains, and I have actually kept most of the gains even though I've been off of Anadrol for a month, I think that would be attributed to, me being far from my potential, me minimizing gains in water during use (lowered increase of strength gains of cycle though) and the fact that post cycle I have been doing strength specific training which is not near as hard with protein catabolism as bodybuilding, and my cns has probably increased my strength at a slightly lower rate than my loss in muscle diameter to counter react the leveraging disadvantage I now have.
I think Anadrol gets most of it's strength gains through water manipulation just like dbol does. I would expect anadrol to give some cns related gains, but not as much as most would believe since anadrol is by far the weakest steroid when it comes to it's binding affinity to the androgen receptors.
In terms of which steroid makes you stronger, like I said before beastdrol is stronger on a mg to mg basis (most likely because it binds with the androgen receptor better than anadrol) but you can take a higher dosage of anadrol. I'd bet that anadrol has the ability of giving better strength gains on cycle and beastdrol letting you keep a higher percentage of your gains off cycle.