What monitor do you suggest for Amiga 600/1200 systems?
Well, I'm kind of a fan of the original 15/30khz multi-sync monitors for an A1200 (probably not a lot of benefit for an A600 there, though).
The link you posted (to a 1084S) is a nice monitor for the A600 or gaming rig A1200. It doesn't support the A1200's Productivity/DblNTSC(PAL)/Super72 modes, though. (Very few games use those, but they make for a very nice stable non-flicker workbench).
I ran a 15khz monitor (similiar to the 1084S) on my A500 for quite a few years. Perfect for games, but flickers a lot in workbench, if you use an interlace mode (which you pretty much need to, if you're going to do anything in workbench, you need that extra real-estate).
I ran a 15/30khz multisync on my A1200 for most of the time I used it, and was very happy. Great for games, and rock-solid workbench modes. IMHO, the best of both worlds for a retro experience.
I run a ScanDoubler on my A4000. Works great, and gives excellent compatability with modern monitors. More expensive overall, and sometimes when de-interlacing video, you'll see strange scanlines drawn in high motion areas. Just the nature of the beast.