Most obviously, each is based on a movie. GHOST is based on the 1990 Hollywood blockbuster movie starring Patrick Swayzee, Demi Moore, and Whoopi Goldberg and directed by Jerry Zucker. ONCE is based on the lesser known low budget 2006 Irish cult movie written and directed by John Carney.
Both are love stories in which (spoiler alert) boy does not get girl - and for equally bittersweet, gut-wrenching reasons.
Where the similarities abruptly end is in how their directors and design teams have decided to present those stories to the audience.
GHOST is a never-ending visual feast of stunning projections and visual effects that keep the audience oohing and ahhing in surprise and amazement. At the beginning of the show, images are projected onto a huge scrim at the front of the stage - complete with the title of the show - making you almost need to check your ticket to make sure you didn't accidentally walk into a movie theatre rather than a Broadway show. Huge scrim-based walls slide in and out to create the basic shapes for the various locations of the story, while a constant barrage of fast-moving projections adds the details. The able ensemble contributes extra flair and color with many flashy dance numbers - augmented by larger-than-life colorful disco-style silhouette versions of the dancers projected behind them.
The score, by Dave Stewart and Glen Ballard (with additional lyrics by bookwriter Bruce Joel Rubin) is that driving, showy, contemporary pop that has become very popular in musicals these days, giving the singers lots of opportunities to bring down the house with their bravado.
Despite the talent of the cast, the real star of the show, of course, is the series of visual illusions used to accomplish all of the moments we remember from the movie - ghosts walking through doors, moving objects through the air, taking over other people's bodies, and being sent over to the 'other side' either by the white light of the angels or the red glow of the devils. Oh, yes, and a brief, almost cursory nod to the most famous scene of all with the pottery wheel (which presumably doesn't get as much focus because it doesn't take a visual illusion to accomplish it).
In ONCE, there isn't a special effect to be seen that isn't created by the singing/playing actors, with a little help from some simple but effective lighting. The set is an Irish pub (where audience members can actually buy beer and listen to live music before the show and at intermission), and the ensemble doubles as the band. Or, the band doubles as the ensemble. They're all so good at both, it's nearly impossible to make the distinction.
The various locations are created in front of us by the ensemble with the slightest shift of furniture, and aided by defining patches of light. This minimalist approach is broadly acknowledged by Enda Walsh's script, as when Girl suggests that Guy join her at a music store operated by a 'big man'....and then, as a tall man from the ensemble wheels a piano to center stage, Girl says 'Here's the store and this is the big man' and we are instantly there.
All of the songs in the score are 'diagetic' - meaning there are no characters breaking into song to express their feelings. Instead, the characters are songwriters who are actually writing and performing songs they have written to express their feelings. (All of the songs are written by Glen Hansard and Marketa Irglova who also starred in the movie.) (Make no mistake - I'm absolutely not suggesting that characters shouldn't burst into song - where would musical theatre be without that conceit?- but I do admire that this show doesn't try to navigate the dangers of combining both kinds of songs in one score. Not that that is impossible, just...tricky...and rarely worth it.)
The multi-talented ensemble provides energetic, toe-tapping, infectious accompaniment, along with strangely beautiful choreography, and an array of colorful, eccentric and endearing Irish and Czech characters. The music is contemporary, but haunting, acoustic, percussive, and, above all, fresh.
I would quite honestly recommend that you see both musicals as they are both highly accomplished and entertaining with great casts and production values. See GHOST for the breathtaking special effects, even though it might leave you a bit emotionally disconnected. See ONCE for a unique and compelling theatrical journey probably unlike anything you've experienced before.
And then decide for yourself how you weigh in on the fact that GHOST was nominated for three Tony awards (but NOT for Best Musical) and didn't win any; whereas ONCE was nominated for eleven Tonys and won eight, including Best Musical, Best Book, Best Actor, Best Orchestration, Best Lighting, Best Sound, and even Best Scene Design (too bad for GHOST that there is no category for special effects). Apparently the Tony voters are not blinded by glitter. Only time will tell if the audiences are.
Spectacle is very popular on Broadway today and it certainly has its place (and I really do enjoy seeing it). But, in my mind, STORY will always win out over everything else because it is only the story that has the ability to capture not only our attention, but also our hearts.