Much depends on your "home base" and personal goals: available storage space, viewing area(s), path(s) from storage to your viewing area(s), physical strength, tolerance, etc. If you have plenty of room for an 8-inch Newtonian, and additional mass/bulk will be no problem, then you may wish to save a little more $ (spend a little more time) and bypass the 6-incher in favor of the bigger scopen. There are lots of folks who have 6-inch Newtonians and are not "itchy" to get a bigger scope. On the other hand, there are lots of other folks who spent a few nights with their new 6-inchers and realized: "Well, dang: now I want more!"
The Heritage 150P (or "Tabletop," here in the U.S.) has interesting features: collapse-able for storage, nice price, et.al. However, many folks find that a "table-top" mounting is indeed sometimes handy, but often lacking. A "two-armed" Dobson-style mount (rather than a single-armed mount like the Tabletop's) will be more rigid & stable. The single-arm design may be fine and stable enough, but a traditional two-bearing altitude axis will be much more rigid, balanced, and capable.
A new "classic" 8-inch Newtonian (Sky-Watcher Classic 200P, Orion XT8 Classic, Zhumell Z8, Apertura DT8...) will be priced around $450, but - as you know - you may have to wait for stock. "Deluxe" versions or additional accessories will nudge the price upward. A second-hand 8-inch classic will cost even less - often less than $350, sometimes a little less than $300. Again, you may have to wait. -And how about this:... considering the "new" cost of an 8-inch classic - a second-hand 10-inch classic can be found for a similar price (~$450).
My own recommendation is to, first and foremost, consider your "home base" logistics and all those ancillary factors of owning - and USING - a bigger telescope. If your primary limitations are budget and space, then a 6-incher may be a wonderful bargain. If you have the space and the wherewithal, then an 8-incher may be perfect for you.
Best wishes and luck.
Edited by MisterDan, 12 June 2021 - 01:07 PM.