Both will be great for attacking. With high withdraw odds it sometimes does not matter a city has hill defences as you know the first few attackers might withdraw.How often do Ai not have metal defenders? Once they get metal units archers are produced a lot less. You will always pick your target in any way. Chances of protective Ai are unlikely.
As Mylene suggest the sooner you declare the easier the war should be. With cha NC can get 3-4 promotions easily. Only 1-2 attacks if you have stables and barracks. Barb units really help too.
Withdraw is nice if you have a lot of attackers. But say you're trying to take a heavily fortified city with 3 defenders and you have 6 Numidians. You may fail in that case but a group of stock 6 HA's will probably take the city. If you have 10 attackers then it doesn't matter but then you have overwhelming force anyways. I agree with Fippy that on Deity Numidians are probably better because AI hook up metal so fast but on lower levels, stock HA's are better. And this data kind of corroborates that. Numidians are marginally better against Spears and significantly worse against Archers. On Emperor/Immortal rushing with HA's I rarely ran across that many Spears and I'm not so efficient like a lot of players on here so I probably rush a little later. Like you said though I pick my targets and wouldn't go after Zulu or Maya for example.