As been said if those are the only 2 choices of course we are all going to say the DJ then the sound gear,,, we all know that by experience.
Which is why sometimes taking a stand and not using a sub-par system would not, or should not reflect bad on the DJ when using it because they can play the right mix of music, still read the crowd and know how to jump start any party.
It would reflect bad on the DJ who said no, I won't use it and walk out if a bar or hall happen to have a bad system.
For example, lets say that a DJ was hired by a B&G to do their wedding day outa town, and the DJ was told that the venue had a system, with mixer speakers and the whole deal and all the DJs use it. It's fine.
So now you drive to the event, without your gear because, ya know, they said it had a system, and you get there and you walk in and it's like "OMG! WTF is this piece of crud system!"... maybe the tweeters are blown and one of the 15's in one speaker was blown and the DJ the night before spilled a drink in the mixer and now the faders sound scratchy.
So now what do you do is,,, the system works, we know it's not the best sound, but with the skill of the right DJ, they can pull it off, (after of course explaining to the B&G about the system, hopefully in your pitch you didn't mention that you bring back up gear, because THAT is what you should have done)...
now the DJ sneaked by, made the party work and if you think about it, taking a job, any job, a wedding, school dance, club should require the DJ to go into with the same amount of professionalism no matter what the pay.
Why? Because it was the DJ's choice to take that event at that pay. Because we choose to take an event that is cheaper , that does not mean we can go into thinking they don't get 100% of our skills as DJs.
If we do a $2000.00 wedding or a $150.00 club gig, each event deserves the best that we can offer as DJs.
I've done a lot of work with "show bands" and as I watch them, they will do the same high energy show if there are 3 people in the room or 3000, the drummer will still rock if they bring him a 3 piece set or a 12 piece set.
I take inspiration from acts like that. give 100% all of the time, make every event rock to the best of your ability no matter what the circumstances.