velox or aventum?

Your question is excellent. The shipping cost is not, in my opinion, a deciding factor. With the Aventum, the system must be shipped freight due to its weight. I chose freight for my Lumos, simply because I didn't want to have to worry about unnecessary damage along the trip home. That cost was actually absorbed by the discounts Digital Storm provided me for going above several thousand dollars. Not being coy there but I can't remember the threshold. For my $5000 Lumos, DS's discounts covered the freight shipping.

With that out of the way, I look to capability of the platforms in question. For my "training wheels" Lumos, I opted for a single 360mm radiator and custom liquid cooling for the CPU. I know now that I will need to get a 240mm radiator in the near future for water cooling the GPU as well. All on the same loop; just a couple of extra tubes, fittings, and GPU water block.

I mention the Lumos given its size. I won't be able to fit two 360mm radiators. Not sure the Velox would provide much more room (length dimension). The Aventum clearly has radiators on steroids. I think there are three 2 x 360 and 1 x 480. Clearly way more cooling capacity than I could conjure up with a 3950X and a 200 Super.

Looking into my future crystal ball (beyond the Lumos service life), I am anticipating an HEDT build (i9x equivalent), and a 2080Ti equivalent. Running a quick play-through of the EKWB loop configurator, I'm still looking at less than 700 watts of heat. The Aventum would certainly keep my system cool. But, the Aventum would be overkill for my needs.

I would need something between the Lumos and the Aventum. I've been studying some completed builds on PCPartPicker. My understanding is that Digital Storm can build a completely custom system outside of the "catalog" if we supply the parts and pay the labor. Instead of overkilling the problem, or settling for underkilling, we can find a middle-ground. This is assuming you are not willing to build your own system. Something like this (courtesy of calibur45) looks rather straightforward:

Enjoy.

Upgraded fans won't matter much for a custom loop build. The most important parts that generate heat are under water, so the need for airflow is reduced. The case is highly rated for good airflow at stock. spend th money only if you want the look.