Well, it has been some time since I've last played GW, so these are my opinons.
To choose the combination that best suit yourself, it boils down to your playing style. All of them includes the 6 core classes, but Factions has the Assasin and Ritualist while Nightfall has the Paragon and Dervish. You may want to read up on those 4 classes before you make a decision.
http://eu.guildwars.com/gameplay/cat...e_professions/
It's usually difficult to complete the later missions alone. However if you are unable to find players to party with, you can choose AI companions called Henchmen to fill up your party. While the AI henchies are relatively competent, they will still make you pull your hair out in fustration sometimes.
Flashback to one of the last few missions of Prophecy -> AI:
"HALP! IM STANDING ON LAVA WHILE ENGAGING THE MONSTERS. IM BURNING! HEAL!" *dies* *gets ressed* *repeat*
If you rather play missions yourself with only AI, then a feature in Nightfall (
and EotN) might interest you. It's called Heros. They are like your regular henchmen except that you can customize their skills and items as well as having more control over their actions.
Do you PvP alot? Factions has Alliance Battles (
12 on 12 to conquer new territory for your faction) while Nightfall has Hero Battles (
1 on 1 matches, supported by 3 of your Hero NPCs)
You can jump, but it's only aesthetic and doesn't affect movement. There are slopes, stairs, bridges, high and low ground, if that's what you were asking.
You can refer to GuildWiki for more information.
http://gw.gamewikis.org/wiki/Main_Page