Paid vs. Volunteer

With a good hiring process, paid staff is always more reliable than volunteers.

Which is better?

Neither, however here are some rules of thumb:

Would hiring someone to do a previously volunteered job better achieve your mission? Can one person do more than a series of different volunteers?
Start by hiring people to do volunteer tasks that no volunteers want to do. Examples: early morning shifts, accounting, dealing with kids, grant writing, volunteering coordinating, cleaning, etc.,...
If you have more than enough volunteers to do a task, you don't need to pay someone to do the job, you need to pay someone to coordinate the volunteers.
As soon as you can afford to hire someone -- do it even if it is only part-time to start. Not only are you making sure that the task will get done (because if it doesn't that person might lose their job) you are creating jobs. Not just a job, but an amazing opportunity for someone to make a living doing bicycle advocacy, getting more people on bikes more often.

Jealous Volunteers

So that no one gets jealous, make sure your hiring process gives everyone an equal opportunity and decisions are solely based on qualifications -- not friendships. If conflict of interest may be an issue -- you aren't the one that should be making the decision, find people that aren't bias.

Prioritize Job Tasks

Like a business you have to grow before you can create jobs, often one at a time. Consider AmeriCorps as an option to pay for most and sometimes all of their pay.