Being able to use the advantage mechanic before or after the roll is known doesn't affect probabilities. It does, however, affect efficiency of use, so you're get better use efficiency out of rolling after, but you really won't change the probabilities of success from rolling with.
The probabliities involved are heavily dependent on the roll needed. As such, there's not general solution. Instead, you have to look at use cases to get a feel for how each works at different break points. Let's assume the easy/medium/hard (DC 10/15/20) breakdown. Let's also assume level 5 characters with a +6 proficient skill and a non-proficient +0 skill and compare the two at the above DCs.
DC.|.Pro....+d20.....+d6.........Non.....+d20....+d6
10...85%...97.8%...94.2%.....55%....79.8%....72.5%
15...60.......84.......77.5..........30.......51.........47.5
20...35.......57.8.....52.5.........5.........9.8.........22.5
As you can see, the margins matter. +d6 is the better option when you need to roll 17 or better, otherwise, rolling a second d20 beats the odds every time. This is because, over time, rolling the d6 every time you're within 6 of success (it doesn't matter otherwise and doesn't add) nets (as stated previously) a 17.5% increased chance of success. When rolling a second d20, the odds are increased by 18.8% at 16 and 16% at 17. So, 17 is the break point where the d6 will improve chances over the second d20. At level 5, NONE of the DCs meet this for the proficient -- they're always strictly better off with the second d20. However, for the non-proficient, there's huge increase in chances as they approach a more difficult task. This may be a behavior you wish to encourage -- the non-proficient have a improved chance to succeed at progressively harder DCs, but I find it a bit counter-intuitive.
In short, the +d6 has very distorted impacts at the very low and very high ends of the scale, and moderate effects in the middle. The +d20 has a fairly smooth curve of effectiveness, very high at low rolls, to good in the middle, to a low impact at the high end of the scale. This is for the d20 roll, though, you need to factor in what roll bonuses look like at your chosen play level because that will definitely impact the use of these. High bonuses make the d20 very much the best choice, Low (or negative) bonuses gain a great deal from the +d6 at the high end, but not so much in the middle.