I am a supplier to both and Staples is better to deal with.
You are absolutely correct about competition if this deal is approved. The FTC is working with Staples to sell part of their B to B portfolio so that there will be competition available for people like you. However, from what I have read in the papers once the deal is approved your chioces will be Staples and a competitor about 10% of the size of Staples. So not really much competition.
My suggestion - lock in a long term contract now while you can.