When I chose my MK67, some years ago, one key reason was its design -- compact and relatively lightweight for its aperture. Another key aspect was its much-lower cost, relative to 4-inch apochromats. In terms of optical/visual advantages over such apochromats, the MK67's are admittedly subtle &/or minor (e.g. angular resolution). In other words, an MK67 or MK66 will not demonstrate a great and easily-visible advantage over a 4-inch apochromat. The apochromat's own advantages involve low-level contrast and hue saturation. Relative to a typical 4-inch achromat, the MK67's advantages are a little more obvious: no chromatic aberration, likely better optics, angular resolution). I consider my MK67 a viable "substitute" for a 4-inch f/12 apochromat.
One important consideration, of course, involves thermal conditions. Sharply falling temperatures will prove more problematic for a close-tube cassegrain. Likewise, a 150mm Maksutov will require more time for ambient cooling of its mass -- particularly its primary mirror and the secondary baffle tube). I'm sure you're aware of this, but I thought I'd mention it nonetheless.
Obviously, the advantage in bulk/weight is significant. In this regard, a new MK67 or MK66 may be a great alternative. (Incidentally, I'd likely choose the MK66 over the MK67, due to its more-versatile focuser mechanism. The MK67 is not as compatible with binocular viewers.) If, however, you're quite interested in a more significant optical advantage (one which is more readily visible at the eyepiece), a larger cassegrain may be a better choice -- either a 7-inch Maksutov or perhaps even a Celestron 9.25 optical tube assembly. A new 7-inch Mirage or Intes Micro is certainly more expensive (& bulkier/heavier) than an MK67 or MK66. The same applies to the C9.25. However, the additional aperture is certainly an advantage when conditions allow (good seeing). Excellent second-hand samples of 7-inch Maksutovs or Celestron 9.25s may indeed fall within your budget. Alternatively, you might consider a new 9.25/CG5 combination and perhaps offset some of the expense by selling one of the mounts.
If the choice and budget were mine, I would try to find a second-hand 7-inch Mirage or Intes Micro, or a Celestron 9.25. If that search proved fruitless, I'd likely choose the MK67 or perhaps the Intes Micro MN56 (Maksutov-Newtonian). While not a large leap forward in aperture and visual impact, either Russian Mak would certainly please your eye when viewing the planets. In either case, the EQ3 would definitely manage the MK67 or MN56 with greater ease.
Cheers & best wishes.