1. The new Instax Neo. Pros: finally a serious camera from Fujifilm. Cons: Small images. No monochrome.
2. Instax 210. Pros: Nice sized photographs. Great color. Cons: A joke of a camera, no monochrome.
3. A Polaroid Land camera: Pros: both color and monochrome. Good image size. Cons: Old camera, hard to find in China, questionable reliability.
Tough call.....each option has an appeal to me.
You could also shoot instant in a "serious" camera. Peel-apart Fuji in my Speed Graphic is a lot of fun, and a lot easier than stuffing Instax into 4x5 holders individually.
To me, neither the equipment nor the film define "serious" photography; rather it is the intent of the photographer and the images produced.
In 2006 I was in Yosemite and had my SX-70 with me. I made a photo of El Capitan and of Half Dome that impressed me with the creaminess and tone of the colors - right then I knew I had two keepers (which I later scanned and enlarged to 8x10). These two are among my very favorite photos that I've made over the last 30 years with various formats and media. I suppose if I'd used 35mm, or whatever - even digital, I might be able to tweak the results to look similar -- but I knew the shots had an amazing look right after I took them.
(I will disable my signature after 20 posts) Nikon: F, F2 x3, F2S x2, F3/T x2, F4S x2, FM, FM3a, D700 Canon: AE-1P, RebelG x2, Elan 7NE, QL17GIII, Canonet 28 Leica: M3 x2, M6 x2 Exakta: VX x3 Hasselblad: 500C/M, 501C, SWC, 553ELX Mamiya: RB67 Pro S x2 Polaroid: SX-70, SLR 690, Image 1200 Other: Pentax SP500, Ricoh GR1, Minolta Maxxum 7, Graflex Pacemaker 4x5, Fuji X-Pro1 Lenses: way too many to list