Both the Silver Bullet and the Maxi Roughstack will do 6v equally well. The manufacturer of the SB doesn't recommend using it at 6v for liability reasons while Madvapes takes a less cautious approach. The reason Altsmoke doesn't recommend it is that when you stack batteries, as you're required to do to obtain 6v, the chance of battery failure increases. Li-ion batteries when used improperly can explode. The use of unprotected batteries in either device can dramatically increase the potential for catastrophic failure. This is the scenario that Altsmoke is trying to avoid by not recommending the use of their device for anything other than 3.7v.
As to which one to recommend, it depends. The Maxi Roughstack is cheaper, and it works. The Silver Bullet is built much better. The fit and finish is excellent, while the Roughstack is a bit more crude. There are two main drawbacks to the Roughstack that I've found. First if you get leakage from your atomizer, liquid can get inside the Roughstack. Second the inside diameter of the tube is smaller than on the Silver Bullet. Some 18650 batteries will not fit for this reason. I have 3000mah 18650 Ultrafire batteries for use in the Roughstack and they work. I also have some 2600mah Ultrafire high drain 18650's for use in the SB. They don't fit in the Roughstack because they're slightly larger in diameter. I purchased the Roughstack before I knew the difference between batteries. The 3000mah batteries are low drain, while the 2600mah batt's are high drain. The difference in performance is quite noticeable. The high drain's produce more vapor and do so more quickly. This is because the batteries were designed to release energy more quickly.
Given a choice between the two, the Silver Bullet wins hands down. I used the Maxi Roughstack as my primary device for months. As soon as I received my Silver Bullet the Maxi Roughstack fell to backup duty. I haven't used it since.