It's really going to depend on what air tools you use and how you plan to blast. It is probably easier to buy the 7.5 HP and smile than pay a lot for a 5 HP and wish you bought the 7.5 HP instead.
I have die grinders that use around 20 CFM and angle grinders that are rated at 35 CFM. For angle grinders, they use the most with a grinding wheel. With a cutoff wheel through thick steel they slow down enough that even a 5 HP could keep up. Air drills, if you using those at full speed, can also use a bunch of air. I am not experienced in sand blasting. Some people seem to do OK with 5 and others prefer a 7.5.
Another vote for large air storage. Some of us on here have 200 or more gallons. By putting the tanks in series it also dries the air without the need for after coolers or air driers. I would vote to use your existing 60 gallon for this purpose, iff the data plate says its rated for 200 PSI. Some of these single stage pumps are on tanks that are not rated high enough to safely be used with dual stage compressors. If you wanted only a single large tank, tank size does help a little bit with dropping moisture, but I'd still want to have an after cooler package. They do sell 5 or 7.5 on 120 gallon though its not as common.
I have 2 x 5 HP on 240 gallons but because I live at 6500 feet, it performs like a 7.5 HP unit (downrate for altitude). This is enough for my needs on all the tools I listed. The extra air storage means the compressors don't come on that often using impacts and stuff like that. A couple tire rotations/ brake jobs before they're ready to come on. I can also do cutoff wheel work like cutting a collar and bearing off an axle shaft without them coming on at all.
Good 5 HP units will make around 17-18 CFM and good 7.5 HP units will make around 25-27 CFM.
Going to a dual stage setup is also a huge leap over your 3 HP 60 gallon single stage. They operate at 140-175 PSI so not only do you get more air volume in the tank, it also eliminates compensates for pressure drop across air lines and couplers, you're air tools won't be down on power. I have my regular set at 112 PSI using a 25 foot hose so the tools see around 90 PSI working pressure. Imagine on a single stage compressor with a 50 foot hose, the cutin is 90 PSI so your air tools is probably seeing around 60 PSI when the compressor is ready to come in... and even less than that if you've got those little swivel fittings in your tools.
The lowest cost thing you can do is simply buy another 3 HP / 60 gallon compressor and run both at the same time. But you won't get any of the dual stage benefits.
