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Imperial City has a much higher percentage of NPCs, Minibosses, and World Bosses. However, as long as you can handle those, the real threat always comes from other players.
My recommendation is to bring repair kits. The exp from killing NPCs will damage your armor much quicker than in Cyrodiil and its harder to get back to base to repair with a friendly NPC.
Cyrodiil will be best for transmute crystals. You can earn 50 per month at the end of the campaign if you reach the tier 1 rewards, or about 25k AP. There's a lot of ways to make AP, so you could solo resources or follow a big group of players around to different objectives.
Tel Var is a currency that's specific to Imperial City. You gain more from NPCs when you carry more on you, but you'll also lose half of what you're carrying if you die to an NPC or player. If you kill a player, you'll get half of what they were carrying. You can use it to buy various items from vendors in your home base, so you might look to see if you need anything from there.
Imperial City tends to have more tanky world boss farmers hunting for Tel Var and stealthy ambush gankers who hunt the Tel Var farmers.
Cyrodiil has a much bigger variety of players. If you can find a group of players to tag along with, that might make it easier.
Imperial City gives more gold per second than any other zone if you know how to play it.
-Key fragments sell on traders for 500 per. A stack of 60 will sell for 30k, and that count is deliberate since key fragments are consumed in multiples of 6.
-Hakeijo runes are used to make prismatic defense runes. Those cost 5k tel var, and usually sell for 60k+. Sadly, they've been selling slow lately since the late game pvp community that uses those is going extinct.
-Collectibles and motifs can sell for a large amount, since it's a way for pve players to collect them without pvping. Prices vary on those more than the former 2 examples though.
You leave base with 1k telvar, farm 2k+ tel var, queue for no cp cyrodil, queue for ic, bank your stones, repeat. There are nice dense clumps of mobs in the sewers that you can bomb for fast tel var, and most gankers would rather lurk around the Elven Gardens daily quest spots since that's easiest daily to catch people doing.