I know you are just asking which one is easier to learn but this is the advice i usually give people who ask me about whiplashes. I am sure the topic of whiplashes has been covered on here and i know TJ gives tons of tips. So take this for what it's worth and it should help.
It is true that if you learn steamrollers (regular handed holding the seat in a straight line) that this is definitely easier than even a double whiplash and possibly easier than a single whiplash (although that's debatable).
If you ultimately want to learn multiple whiplashes (brakeless i assume) then it is advisable to learn steamroller and hang five on both left and right foot. Because when you do whiplashes (depending on which side of the bike you start on and which direction the whiplash is going, you will require some balance on both feet. The reason i say this is because whiplashes can be broken down into what i call mini transitional hang fives and steamrollers (without holding the seat of course).
So if you are starting with your body on the right of the bike and kicking the frame around clockwise you'll be starting off balance on your right front foot (which unless you're an opposite rider) this should be your weaker rolling foot/side. So you want to strengthen the balance on your opposite foot so you maximize your stability on the initial kick around. Of course when you kick it around you'll follow your left foot directly to the left front peg so you can prepare to finish the single whiplash with stability on your good left balance foot.
Of course for multiple whiplashes you'll kick it around and then as you step over the frame you'll be entering a temporary hang five position, hence why i describe whiplashes in terms of "steam-five-steam-five-steam-five etc."
So i would practice a plain steamroller and a plain hang five on either foot,especially if you want to eventually do whiplashes in either direction.
Another couple tips for multiple whiplashes are: as you kick the frame around shift your body position to where you are hovering over the bars and establish somewhat of a locked in position and try not to chicken out or stray from this basic hovering position because you'll need that consistency as you transfer balance and step over the frame. Also, do not hesitate in your stepping. what i mean is don't wait until the last second to transfer a foot to any peg because you'll feel rushed and be playing a game of catch up and never really feel comfortable. Holding your grips near the ends of the bars (even overlapping the bar ends a bit) will give you better tilt control as you shift your body weight from side to side. Extremely narrow bars are generally not good for brakeless whiplashes but i'm sure some riders started with narrow bars and learned them. Don't lift your feet behind the seat on multiple whiplashes because that's all the further you feet will have to travel to get back to the front pegs where you need them to be. Top tubes are so low nowadays that you'll have plenty of time to quickly lift your feet and get them back to the pegs for maximum constant control of the whiplashes.
for a steamroller just kick the frame around and take your hand off to grab the seat just before it gets to midway point. if you take your hand off too soon like at the same time as kicking the frame around then you might destabilize before you get to the steam. Just some thoughts and not saying it's the ultimate or correct advice for everyone.
I'm not saying you have to learn every trick opposite and regular (that was for the few of us back in the early 90's when tricks were not as cool or hard..haha poke poke jab jab) but it can definitely help open doors for creativity and expanding your tricktionary in general.
Also, two footed steams may help for whiplashes like the other guys said, although whiplashes are not really a two footed rolling trick and rolling two footed will not focus as much attention on training each individual foot for maximum balance. I would bet that many riders who can do two footed rolling tricks can't balance/roll equally on each foot when rolling on one's opposite foot is required.
Don't know how much more i can post for a while but i wanted to help a bit.
Good luck and skill buddy,
chase.