I tried it both ways. The wall of toilets worked better for me because she could go down the row and use each one until it broke. She didn't have to wait for it to finish flushing, just sit and run. Her first pass might be 1-2-3-4-5-6-7-8-9-10 but by the fourth or fifth run, several of them would be broken (and they'd all be seriously cruddy!). She'd keep going, skipping the broken ones until they were all busted, then repair them all in rapid succession.
When she used only one, it seemed to take longer because she had to wait for flushes to finish and could only repair that one at a time. It probably worked out just about the same but the wall *felt* as if it went faster.
One of these days I'll do a serious test. Wall of toilets, wall of sinks vs singleton sink and toilet. Heh my poor Sim!