Unless it's a new-school ultra-fine blowing wool like JM Spider or Certainteed Optima blown at the right density it's real-world performance will be underwhelming, particularly in an open attic blow. Low density fiberglass loses SIGNIFICANT R value in a horizontal application where the cold side is up (like your attic in winter), since air can convect relatively freely. When it's way below freezing in the attic (say 0F) it can be delivering only half or 2/3 of it's rated R for the installed depth. Cellulose (even at ~1.5lbs nominal open-blow density) is far more air-retardent, and the R value remains stable (it actually rises very slightly, but don't count on that in your modeling.)
Under a hot 125F roof deck in the blazing sun again, cellulose will outperform most fiberglass. Fiberglass is semi-translucent to infra-red, and the aborption into the fiberglass layer makes the HOTTEST part of the fiberglass an inch or so into the fiberglass, where it rises well above the attic air temp. So in effect, you're insulating against a higher temp, with an inch or two LESS insulation. Celluose absorbs all of the radiated heat in the first 1/8" and re-radiates most of it back. The hottest part of the cellulose is at the top surface, where it gives up most of it's heat to the attic air, and runs at only a modest elevation above attic temp.
The only real advantage fiberglass has over cellulose in an attic application is R/lb. Cellulose is inherently denser, and if you have skinny joists or the insulation is being supported by extra thin sheet-rock with joists on 24" centers or something the additional ~1.2-1.8lbs per square foot of that 12" cellulose (over 2lbs/ft with your existing cellulose counted) vs. roughly half that for fiberglass. If loading isn't an issue, go with cellulose, and insist on "stabilized formula" if it's to blown initally to 12", or "borate only" dry blow goods with sufficient excess to stabilize at 12" over the next decade's worth of settling. The manufacturers spec out what that is based on the blower settings and the product- typically for 12" of settled depth you'll need 13.5" on day 1. Fiberglass installation has a similar factor, but it's easier to screw up and over-fluff fiberglass (to ill effect) than cellulose.
If you go with fiberglass, going higher-density (1.8lbs/cubic foot) new-school goods with a "blown in blanket" mesh on top pretty much eliminates the fluffing, sagging, and convective loss issues, as well as the IR-translucency problem. But BIB is a more expensive way to go. It's usually cheaper to just heap on more cellulose and forget about it if you have the space. The tricky part is getting the R-value up at the soffits where you might have to thin it out to maintain attic ventilation. It's often worth the money to use closed cell spray foam (R6/inch) or stacked cut'n'cobbled rigid XPS (R5/inch) sealed with 1-part foam on top of the studawall plates, and to form the chutes between the rafters for maintaining ventilation. If 18" thins out to 7" at the edges, the thin wedge of insulation at the edge starts to dominate the heat transfer. An 18" of settled cellulose is good for about R65, but 7" (as with 2x8 joists) would only hit ~R25, and every square foot at the edges would pass ~2.5x as much heat as a square foot of area out in the center. It's enough to show very distinctly in IR imaging. But 7" of 2lb closed cell foam would be ~R42-R45, which isn't nearly as large a differential.