To engine or not to engine?
Read only if you've really got nothing else to do.
Riding a bicycle across countries is a great way to see the world but it does have its drawbacks. Would a motorbike be a better choice of transport? One of the great things about a bicycle is that you travel slowly so have a chance to actually see and experience the people & places you'll pass through.
One of the major drawbacks is that it takes so long to get anywhere. Isn't that the point though? Yes and no. However interesting a country is, it is bound to still have some relatively long & boring stretches of road to cover to get you to the next good bit. One solution is to put the bikes on a bus/train/boat. Once you accept that this isn't cheating (it's just a better use of your time & effort for the reward) then this works quite well.
Motorbikes (even small ones) can cover ground so much more quickly. On a big one you could blast out hundreds of KM a day but that's a whole different type of trip altogether, I'm therefore going to think of a motorcycle only as a little C90/scooter or max 125cc which will naturally limit your speed.
The little villages we've been riding through on the bicycles have been lovely. Kids shouting out "hello", chickens crossing the road, dogs lazily getting up, stretching moving out of our way. At a speed of say 15km/h you get a proper opportunity to look at things and feel (if only slightly) part of the place. But after about 500km of them, we've kind of seen enough...
When we hired a scooter the other day I slowed down from a cruising speed of 50-60km/h to about 25-30 when going through the villages. That's way faster than on a bicycle. You definitely don't see or hear as much but you still do get a pretty good idea of what is going on there and I'd see it as a pretty good compromise of immersion in location vs distance covered. So that's +1 to the scooter!
One way that a bicycle wins hands down is bureaucracy when entering/exiting a country. No customs paperwork, no hassle, just straight in. You could travel on a £10,000 bicycle with no questions asked or on £1,000 worth of scooter and have a whole load of costs & potential hassle. It's crazy, but d'ems da rulz. +1 to the bicycle.
Also getting a bicycle to and from your chosen start/finish point is so much cheaper and easier. It could be as simple as riding to the airport, taking your pedals off, turning the handlebars and heading straight to check in. Try that with a motorbike! So another +1 to pedal power.
A big factor is which country(s) you plan to visit, what you want to see whilst you are there and how long you've got to do it in. Generally speaking, in a fixed amount of time, on a bicycle you will be able to see less places but potentially experience more of the ones that you do get to visit than on a scooter. A 200km diversion to see a castle/waterfall/beach on a bike just might not be possible if you're on a limited schedule (or its on the other side of a mountain range) , where as on a scooter, you could be there in a day.
Developing countries tend to have less roads so you may find that the only option to get around certain areas is the main highway. These can be pretty bad on a scooter but believe me, they are worse by pedal power.
This thought process could go on all day and you'd never get a definitive answer, particularly not one that would be correct for everyone.
After doing a bit of travelling on both forms of transport I think it really comes down to attitude. Presuming you have plenty of time, and if you can mentally slow yourself down, then I think a little motorbike could well be the right choice. But before you head off to Scooters R us with your credit card, do have a quick think about whether you'd feel that petrol over pedals may just be too easy an option. Oh and also check out the size of your beer belly before and after the trip.....