Is Marco's better than Leo's? Interesting question -- best answer is it depends.
In GOTM 37 (almost complete) I had 9 major cities in play (with a couple of minor ones) and really didn't go adventuring until Leo's expired -- thus it achieved little use. No big deal, as I was doing fine with a tech per turn circa tactics & just decided to continue with more modern units (also allowed the ai to build more wonders than I ususally do -- so I had to go out & "collect" them.)
With an Early landing sort of game or an early conquest sort of game -- Marco's retains its value, but Leo's is less important because the palyer is running a trimmer civ and has fewer units to consider upgrading. On the other hand, as Tim pointed out, one may have a bunch of food camels laying around, and building a wonder (any wonder for that point) can free up some badly desired trade -- so Leo's might make sense, especially with some None units in the resource pool.
With an extensive empire requiring a large force of units -- Leo's shines. With an inexperienced player, Leo's is a must.
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What will the ai research next? I don't work that hard, but there is a list of ai preferences (gunpowder is very high for example) based upon thier warlike leanings.
In the diplomat exchange discussion, an option exists to trade maps (about a choice or two below exchange techs.) -- both civs must have the knowledge of Map making, and the maps will be exchanged if the ai is worshipful (almost always one step down and sometimes if two steps down.)
I usually beg once every few turns -- when one is truely pitiful (having a single size one city for instance) feel free to beg every turn.
10s-20s? When I have 50-80 cities, I'll have half that many engineers (and feel like I have too few) and about that many trucks laying around (I often have some extra trucks -- I haven't quite got the knack for Just in Time production/delivery -- so I'll pre build extra trucks as long as the commodities roll, and deliver enough to get a tech per turn.)
Hanging Gardens is a Plus; Mike's is too good to let get away. J S Bach -- similar reasoning -- Should have both by the time you want your towns to buils sewers & become metro gigantic.