I think it depends on the colour copier/printer use choose. Some of the new Canon copiers are mainly B&W but also do low volume full office colour printing. That sounds like what you need.
Your biggest costs in printing is usually cost per copy although people don't see that until it is to late. You ever buy a inkjet/bubble jet printer? I think I saw one that does colour for about $100. I think the cost per copy was $6-$10. Great deal that printer, not. So an office copier cost much more but the cost per copy is pennies and ever less than a penny for B&W. What I'm trying to say is to ask about cost per copy no matter which way you decide to go. If you are doing a lot of printing it adds up fast.
If you get two boxes there is a space issue but if one breaks down the other is still going, if you have one box it takes up much less space but when it goes down you have nothing. In the latter case get a service contract with a four hour responce time, then its not so bad.
There are pros and cons for both sides, good luck.