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Hi cristoo. I went through the same deliberation about four months ago and have just ordered my second TipTop Mantis. I'm extremely happy with the first one. Plenty of power, decent build quality, the Mantis has 208hp vs the Rackbrute's 171hp (with power supply installed). The bottom row of the Mantis is something like 60mm deep, so you probably won't run into too many modules that won't fit. Some of the Doepfer modules are especially deep, so you should probably compare case depth too. If I remember correctly, they both have threaded rails, so you won't have to deal with sliding nuts.
I think the Rackbrute is meant to be a compliment to the Arturia Minibrute, so if you have one of those as an external synth, it may make sense to get the Rackbrute from an aesthetic perspective. Also, will you be transporting your case to gigs? The Mantis is light, but I don't think I would take it outside of my home without a Decksaver cover and a case/padded bag. Aside from these concerns, the Mantis wins in my book.
The only downside I can think of is that I was not initially fond of the vertical configuration for my Mantis. It didn't feel as stable as I hoped, so I have been using it in the 45-degree and horizontal configurations until my second one and dual mounting brackets arrive.
Hope this helps. Good luck and have fun!