1/2 gallon almost doubles your fuel carrying capacity. The key, imho, is what constitutes acceptable cruising range...a deceptively tricky question, loaded with contradictions. That's within the confines of my wheelhouse, I am a contradiction...who rides these bikes. On the face of it, "if some is good, more is better and too much is
almost enough"
Over the years, I refined my goal...downward...to 100 miles. The original target was 150, equivalent to one tankful + one gallon. I've never been fond of auxiliary tanks on the machines, there's not enough cubic real estate. I sourced a 1L G`Craft tank back in `03, a modestly pricey bit of high-quality, functional, bling. Subsequently, I picked up a cheapie 1.5L aluminum tank, that took a full day to metalfinish & polish. Over the winter I added a ~1L stainless tank (that's going to the metalfinishing & polishing spa) to the growing collection. None has yet to be real-world tested. My solution for this, the old dreaded (not to mention fugly & awkward) 1-gallon plastic fuel can, a temporary solution gone rogue. I lived with this setup until 2016. The only upside is that the container could be left off the bike. Seriously, a pair of G`Craft side tanks (they sell RH & LH versions) would be way better, aesthetically...to the right person. I've reached a point where I'm no longer that fond the look of the classic rear rack. Yet, my usage has transitioned to longer distances and to/through increasingly remote locations. The best solution is also the most difficult...utilizing more of the volume inside the frame. There's easily 4L+, fore of the battery & below the seat pan. Developing a tank, with that capacity...that will fit the frame...is, most likely, beyond practical limits. With a 2-piece setup, there's more like 5L potential, along with a set of complications that are tantamount to using scissors to remove coffee stains from fine linen. Quite the Gordian knot, isn't it?
So, where does one draw the proverbial line? Since this has already turned into an editorial, I'll try cutting to the chase. How much time can you really spend in the saddle, without a break? How far will you be from a gas pump? Best edumacated guess, the longest I've ever ridden, without putting a foot down, is ~90 minutes tops, over a distance just shy of 80 miles. Fact is, actual point-to-point average speed (including any & all stops, reduced speed limits, etc) is typically in the high 40s-low 50s. My highest single-day mileage total, 260, a mark that's unlikely to be eclipsed. Some basic math, based on firsthand experience, the stock tank typically reaches "reserve" at 42-46 miles, with an estimated 55-60 miles, total range. With a 3.5L aftermarket tank, massively reworked to fit, actual capacity reduced to 3.35L, "reserve" has come up at 58-62 miles, with an estimated 80-85 miles total range...a disappointing 15-20% shy of my revised goal. But, just how disappointed should I be? 260 miles would mean 4 fuel stops, over ~5 hours (minimum...actual touring would likely mean an entire afternoon and then some), that's separating arse-from-saddle at 75-minute intervals. Now, let's apply those numbers to your 1/2G (1.89L) and 1G (3.78L) auxiliary tank + stock tank combos. The smaller combo equals 4.09L, 73/97 miles; the larger 5.89L with 106/142 miles. That's 3 or 4 refuelings vs. 2-3, depending upon your bikes thirst and your level of range anxiety. You're the only one who can make this call, it's largely subjective.
My suggestion is to take the bike out on the road and really put it...as well as you...through a full-on workout. You already have the rear rack and, presumably a 1-gallon gas can. See how much time & mileage is comfortable enough to enjoy, between stops, as well as your maximum distance from fuel. IMO, 6x10 is a large cylinder on a bike this size. How often will you need this much capacity?
FWIW, my compromise involves replacing the VM22 with an MJN22...which looks like it'll increase mpg by ~10-15%, enough to get dangerously close to the 100-mile goal, with no other changes. For those longer-distance rides, a removable 1.5L tank, attached via a small, custom, mount (in place of the rear rack) should deliver 120 miles+, which I'll call a success. For me, stopping every 60-90 minutes is self-preservation as much as anything else. YMMV