I’ve used the AK and PB vests. There is one in the middle, the SJ vest as well. They are several years old and since then they have evolved and there are other lines as well so I definitely don’t have the current ones.
I don’t think the original AK vest (smallest of the three) even had a place for a reservoir when I got it. I have tried a reservoir in the PB vest in a 100K in 2015 and this year for the same race I used two bottles instead. I chose the Platypus reservoir I looked at REI just for something different, having used Camelbak for years. The CB reservoirs have a large mouth, quarter turn twist opening. It’s the fastest and secure but it’s really large and sort of heavy.
The Platypus is definitely slower to fill because you must slide the clasp on and off to fill but its a little lighter and has less bulk. I do think the clasp could be lighter. It seems too dense for the job it has to do. But it holds up well. It does have some vertical channels that help to keep its form while in use so it doesn’t crumple up when it’s volume gets low.
I recommend about a 50 oz reservoir for the UD vests. Any bigger reservoir is getting too big for the vest. You can supplement with a bottle on the front if you need to. I do recommend a reservoir with quick connects on the bag to make routing the hose quick and easy. Both UD and Platypus have this.
A word about the bite valves – The platypus valve seems to limit the flow too much, particularly if you bite too hard. I ordered some Hydrapak bite valves, which I really like and replaced the Platypus valves. They Hydrapak valve also has a twist-to-turn-off mechanism on the valve, preventing accidental drainage if it gets squeezed. And the Platypus also has some sort of turn off mechanism at the bite valve.
It’s about an even trade off in speed of refill vs convenience to have the reservoir instead of bottles. You can certainly fill bottles quicker than you can remove, open, fill, close, replace the reservoir in the UD vests. The only way a reservoir could be quicker would be if it is integrated into the vest like the wide mouth CB ones. But I want the UD vest and not the CB although I have not investigated all CB vests. There are so many and they change each year.
I even thought about rigging a speed filler and using a detachable quick connect hose end just before the bite valve to fill the bag without removing it from the vest. That is getting a bit complex, though. Could use gravity or squeeze a larger reservoir into the already packed one. Maybe crew could do this while you eat if they had one prefilled to squeeze into yours but it might take too long. Never got to the stage of testing that idea.
With the integrated reservoir as with Camelbak, you get the advantages of speed plus capacity plus ease of drinking but you lose the option to omit the reservoir and use the space for other gear. With the UD vests, you don’t think it will take long but sometimes it takes a while to make the filled reservoir fit properly into the vest with all the zippers, dividers and hose routing that takes place.
When using bottles, I use the small (20 oz) vs the 26 oz bottles in the front of the vest. The larger ones will bounce too much and can even fall out because they have too much sticking out of the pouch. For almost all races, I find that I can use use one 20 oz bottle. My routine seems to be – drink my one bottle en route to the next aid station, fill and drink a bottle at the aid station and refill the bottle again for the next segment, thus drinking about 40 oz per section.
If I had a really long interval between stations or a really hot day, I might decide on the reservoir. I still might carry a bottle so I could have an alternate drink or extra fluid. And it is nice to be able to fill and drink from a container as some races don’t provide cups anymore.