Bondtech kit for 1.75mm or 2.85mm? Doubts

[fergazz 60]

I will upgrade my UM2 with Matheless Nano kit and Bondtech extruder. My doubts are beetwin 1.75mm or 2.85mm. Should be great change to 1.75mm (much easier to find those filaments around) but I need to clarify if the bowden guide will be tight enough and the drawnbacks to get a thinner filament just becase I never work with them. I supose to buy 2 printer soon with 1.75mm, maybe Raised3D and Prusa i3 mk3 (sorry Ultimaker, I need a bigger volume and unexpensive option under U$ 800, I'm sad about it), so it will be much easier to have one type of diameter in stock.

Should be great make a change to 1.75mm but I need to clarify the drawnbacks:

(1) Bondtech 1.75mm pushfit conector inside the adpter fits perfectly without any play? Is it easy to insert 4mm bowden tube all the way into the 6/4 bowden "tube guide"? Both together doesn't leave any gap or improve the friction of filaments during retraction on TFN couper?

doc reference: pages 9 and 10 of BondTech Ultimaker Instalation Guide 1.75 v2.1

(2) 1.75mm will be easier to bend and get a fail print with flexibe filaments?

(3) What about retraction value, what will change beetwin those settings?

(4) More or less pressure than 2.85mm filaments? How it impact in speed settings? Considering that I will upgrade basic UM2 block and nozzle to Mathless Nano kit and race nozzles.

Cheers!