In your opinion, which faction is the strongest in Heroes 5? Let's discuss the advantages and disadvantages of each faction.

IMHO - it's too great a luxury to spend time training peasants when you have a maximum of 20 warriors per week. I turn 20 swordsmen into inquisitors per week. Many prefer turning monks into paladins.

You can afford that when playing against the computer, or on a very rich map...
And in general, actual conversion is not cheap, so do the math yourself:
1) peasants to crossbowmen is approximately 140 kopeks
2) swordsmen to inquisitors is approximately 900 kopeks
3) inquisitor to paladin is approximately 1900 kopeks
Throwing away either 20 or 40 thousand on conversion every week isn't always possible, but 3000 is much more realistic, especially since Dugal's crossbowmen are not inferior in stats to griffins or even inquisitors...