Hi abromowitz,
I don’t know the specifics of the Dunlopillo Harbor Plush and the information they included in the online description is incomplete so if you can find out the information listed here and post it on the forum I’d certainly be happy to make some comments about it. Without more specific information I really can’t make any meaningful comments about the design or the quality of the materials in any mattress.
As you know … I think very highly of Brooklyn Bedding and they are a member here which means I believe they compete well with the best in the industry in terms of quality, value, service, and transparency. The Aloe Alexis uses high quality materials and there are no weak links in the mattress. It can also be customized both before and after a purchase (with rearranging or exchanging the layers) which can also be an important part of the value of a mattress purchase as well (especially online). It also uses Talalay latex in the comfort layers which is a very breathable material and it also includes wool in the quilting so it would be very unlikely that most people would have any issues with temperature regulation on this mattress.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful comparisons between mattresses (once you know the specifics of the Dunlopillo Harbor Plush).
There is more information about the types of support systems that are generally suitable for different types of mattresses in the foundation post here here but as long as the slats in your platform bed aren’t too far apart it would certainly be suitable for either mattress (I would generally use 5" gaps as a maximum guideline for a mattress that has a polyfoam support core but 4" or less would be preferable and better yet).
Again I would need to know more about the specifics of the mattress to make any meaningful comments but with latex comfort layers it’s most likely that it would be fine for most people in terms of temperature regulation. There is more about the many variables that can affect the sleeping temperature of a mattress relative to different people in post #2 here.
There are many good mattresses available in your budget range but the “should and shouldn’t” questions will really depend on your own personal preferences, your budget situation and on which mattress is the best “value” for you regardless of whether it would be the best “value” for anyone else). Since a mattress is one of the most important purchases you can make and the quality of your sleep is one of the most important factors in your overall well being and how you feel each day … I generally suggest that you buy the best quality/value mattress you can comfortably afford and that is a good “match” for you in terms of PPP.
[quote]Me:
Side and back sleeper (mostly a back sleeper)
Weight 140 lbs.[/quote]
There are so many unknowns, variables and personal preferences and individual criteria involved in a mattress purchase that it’s not possible for anyone to make specific suggestions based on specs (either yours or a mattress) or "theory at a distance so I can only help with “how” to choose (following the steps and information in the tutorial post) and help you identify any lower quality materials or weak links in a mattress but the specifics of “what” to choose or which mattress is the best “match” for you in terms of PPP or the budget range you are most comfortable with is really a personal decision (see mattress firmness/comfort levels in post #2 here).
In the end it will come down to testing for suitability and PPP (or a more detailed conversation an online manufacturer if you can’t test a mattress in person), checking for the quality of the materials inside the mattress, and then making good “value” comparisons based on the information in the post I linked earlier in this reply.
There is more about latex hybrids vs all latex mattresses in post #2 here. There is also more about the different types and blends of latex in post #6 here but all of them are good quality materials and the choice between different types of latex is also a personal preference along with a budget choice as well.
There are many days when I wish I could “clone” myself and reduce the hours that I spend (and perhaps spend a little more time sleeping) … but I haven’t figured out how to do that yet so for the time being I’ll have to make do with two hands, ten fingers, and unfortunately … only one brain 
Phoenix