@Rockhead,
Do you mean as in laying them on a driveway, vs building a wall?
Well for me personally, laying them flat would be far easier....being that most of my work in the past 20 odd years has been more on the ceramic tile side of things as apposed to verticle brick laying...really depends on several things, if you were laying over existing concrete....pavers....I don't even have to think about it to lay those...but if you're doing an open patio...sand application...or even the real deal...beaten into a fresh mud bed....then that gets a lot more difficult, the prep work and such.....so I'd say the average person might fair better verticle...vs flat in that case...really depends on how fast they pick up on the brick trowel.
It's all a matter of practice, but working the trowel is prob the most difficult thing to teach someone....you can show them the moves, and that's about it...they have to learn it on their own. Verticle brick laying, leans heavy on proper trowel use...and it isn't an easy tool to learn...I've trained...eh 30 guys probably, mostly in the ceramic tile field, and when I would get a stone or brick job...no matter how good they were with notched or flat plastering trowles...none of them could work a brick trowel for ****. Eh, of course the first time I would screw with them by handing them my trowel I call "Stoney"....he's the big boy, that I use when I'm backfilling...or just moving a lot of mortar...stone work....but once they cried, I'd hand them the normal sized one.