The 78D is a bit nicer finished that the standard 78...but even the standard 78 has gotten a lot better in the last year (at least here in the US). The 78D is worth the extra $ for the fuller sized stock and better blue.
Many of use leave the guns charged when we expect to shoot again in a few days. Some keep them fully charged all the time. Personally, I'll leave it charged and ready to shoot when I expect to shoot again the next day.
For long storage will shoot it down to where it's about time to refill, then store it with that low charge.
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Many UK shooters seem resistant to bulk filling co2. Am told co2 is not as commonly available there as it is here. But with both the bulk fill cap and the standard piercing cap, should be able to make use of either cartridges or tanks, which could be a real advanatge.
To complicate your choice a bit, there are 12gr. adapters that would allow using the 12gr. cartridges on a Qb79 "tanker". Paintball sites have them here for about $13-$18 for the single cartridge model. Co2 is pretty common here, but even so, will often use a 12gr. to do some trigger adjusting, sight adjusting, or just when I want to shoot a dozen times.
