Heat pump for upstairs bonus room?

We're going to have to replace our AC/furnace gas pack unit this spring. It's a 8-9yo Bryant unit, but the coils in the A-coils are leaking and it'll be $2500ish to fix. We currently have a dual-zone unit - downstairs and then we have a 300 sqft. bonus room above our garage. Currently the zone system isn't very effective. The upstairs and downstairs can't run concurrently so it never really stays comfortable up there. Would a heat pump or something of the like be too much? Our current unit is 3.5 tons and we've been told to replace it with the same (1830 total sq. feet). Could we get away with a smaller gas-pack unit if we added a heat pump? Would a better zone system do the job? Do they make them where they run concurrently? We also don't have an "off" setting upstairs so we just have to crank the thermostat up to 90 in the summer and 55 in the winter to keep it from coming on when we don't want it to.

Thanks for the replies. One company came out and measured all of the rooms and gave me all of these crazy charts and told me we needed a 3.5 ton unit and that the upstairs wasn't large enough for a separate unit. If a dual-zone system is suppose to allow both to be on at the same time, we'll look at replacing it (it has clearly been worked on as there was a spare motor and other parts in the crawl space for it). One company blamed it on gravity in that the air couldn't get up the vents. Not sure if I believe that or not. Also, we're in Nashville so nothing too drastic either way, but it sure can get cold and hot.

It depends on where you live and what the most efficient type of fuel is in your area. You need professional advice from a few HVAC contractors in your area. That you can't turn it off in a zone sounds just wrong. Also, I have never heard of a furnace that couldn't do both zones at once. It sounds like it was seriously undersized if it could only hear one zone at a time. Consider a pellet stove or wood stove for your bonus room. The pellets and biobricks (to use in the wood stove in place of split wood) are relatively clean, affordable, and carbon neutral. I heat my entire house with two pellet stoves -- 1550 sq ft + a full basement.

You have too many options to list them here. The place to start is to get a load calculation. This will tell you what size unit you need.