i feel like a little bit of both.
if your jungler is say...Master Yi and your mid laner is Zed. (not a chance they'll be able to peel for you very good)
id say its better to lean more toward mobility than damage , while not sacrificing too much damage.
Now say you have....Sejuani jungle..and Orianna mid..i'd say its pretty safe for you to go a more heavy damage than mobility. because what you lack in mobility, they'll be able to peel.
Overall i think it comes down to your skill with different ad carries you gotta be able to play those different types of ad carries if you wanna keep switching between styles of ad carries.
what the 2 team comps are shaping up to be, you need to be able to survive, it doesn't matter how much damage you do if you just get insta killed, right? If the enemy team starts picking champions with heavy dive/assassination potential on you unless your team counters that with champions to protect you, you better damn pick an ad carry with mobility lol.
Surviving is key. if you're dead you're doing zero damage.
it's up to you to determine how much survivability you can sacrifice for damage and still be able dish out that juicy damage.
In solo queue, when you are queuing up as a lone as carry and the 4 people you get queued up with are random, id rank Mobility ad carries stronger than less mobile ones simply because of the fact that you should not trust these people to peel for you.
I have been in the ad carry position and i just wanna pull my hair out when i don't get peel...i main top/jungle and do my best to peel , so when i see others failing to even try....made me quit adcing xD
anyway back on topic, you can only trust yourself when it comes to queuing up alone.
with that in mind... Be an Ezreal or a Tristana that can peel for themselves.
Survive. Kill things. Climb Elo.
[slayer-jinx-catface] i hope that wasn't too long a response.
Have a good day!!! <3 May the ELO gods be ever in your favor!!!!!!