In absolute terms, but take in mind that life in Spain can be much less expensive than living in Berlin or Paris. It depends on the area and the job.
> I'm currently functional in Spanish but I have a long way to go until fluency. That's one of my goals in moving to Spain
Madrid is the right option then. Has worst climate with more hot in summer and colder in winter, and is as stressful as any other big city, but is a much more easier place to learn spanish, and (probably) an easier place for a foreigner to mix with the locals. Barcelona uses heavily Catalonian language for all: trafic signs, street posters, commerce, casual conversation, television, politics... all.
On the other hand, if you can find work in any of the other "big/but not so big" province capitals (or can do remote work) maybe you should consider it. Smaller the city, worst pay, but much better life quality. The climate is better around the coast, as usual.
> How limiting is it to not be fluent enough in Spanish for use at work
As long as you speak english and know your stuff, should not be a big problem. English is worshipped (an old national inferiority complex) and in fact could be much more difficult to you to find a job in Spain if you were Spaniard.