It's easy to say one or the other for various reasons, but make sure that you seriously do some sitting down with the owner and good research.

Owning an aircraft isn't cheap- and justifying one on paper can be very difficult, particularly for a company that is used to having tangible numbers to work with. You want to make sure that before you commit yourself and your livelyhood, you CLEARLY spell out some expectations for the new owner- otherwise you might find yourself out of a job with little or no notice.

Some things to consider beyond purchase price and the ability of each aircraft to meet the operational goals and needs of the mission:

Crew staffing and training: Is the aircraft going to be a single pilot aircraft? How often are you going to send each crewmember to training? What is the cost of each training event (initial/recurrent)? Will you be flying enough to meet landing and instrument proficiency requirements? Will training be conducted in the aircraft? Do you have provisions for an IPC if you don't maintain instrument currency?

Maintenance: Are you buying new or used? What warranty coverage do you have? Will you enroll in a maintenance management or parts coverage plan if outside of the initial factory coverage? Do you have realistic expectations for unexpected maintenance? What is the cost of inspecting or overhauling the engine(s)? If you're purchasing something used- how much time until overhaul or hot section inspection? Does the reduced purchase price reap benefits, or will you lose money in the long term due to heavy maintenance?

Insurance: What type of coverage does the company need? Can you be insured? How much is it going to cost?

Fuel: Do you have realistic numbers for fuel and other "unfixed expenditures"? With fuel going the way that it is, single engine may be a benefit here (depending on stage lengths, aircraft performance, inflight speeds, etc). How about other costs- has your employer considered the costs of landing/handling fees, GPUs, hotels, rental cars, meals, etc that the crew will need?

What fixed costs come into play? Hangar fees, tiedowns, chart subscriptions, professional services (flight planning, tracking, maintenance planning, etc) are all things to consider.

I can't share specific numbers, but here are some rough numbers to play with from last year at my company. We operate a Citation Encore, but I imagine that most companies follow similar breakdowns of finances:

Percentage of Annual Departmental Costs
*Aircraft acquisition/financing costs not included

Crew Salaries & Benefits: 32.0%
Fuel: 27.7%
Planned Maintenance: 14.25%
Real estate, hangars, associated taxes/fees: 8.0%
Unplanned MX: 6.3%
Training: 5.2%
Data, Communications, Charts, & Other Subscriptions: 2.25%
Lodging, Meals, Entertainment: 2.0%
Travel (Airline): 1.5%
Misc: 0.8%