Hi AY, can only answer for the S-Lab 5 and OMM packs I have. On S-Lab 5 the bottle holders are very stable (though the widest bottles fit may be a bit snug) and hydration pack (1.5L) is great for days you just want some water. And this is a waist and chest strapless vest so it sits on nicely and is held in place by bungee cord clip things (technical term I believe). Great pack, but I think the 12L variation (or similar non S-LAB Salomon packs) are what would be needed for longer events with reasonably-sized kitlists).
I think some of the Ultimate Direction UltraAspire packs offer both too - I'll let those that know for sure answer. And many other packs come with bottles or bladder, but are adapatable to for the other easily (e.g. OMM Last Drop 10, Ultra 12, Adventurelight 20, all of which I've used with bottles.
I will say with the 5 packs I've used for ultra's over the years that you usually get what you pay for with these things, OMM packs are light and practical, but pretty basic and lack front storage options, S-lab 5 (no doubt 12 too) is lots better. OMM packs have side pockets that are semi accessable on the move for two bottles (ok for me, if your shoulders/arms aren't that flexible may struggle), made easier if your purchase the specialist slim bottles to go with packs. The attachments to put bottles on front of packs I found to be pretty useless for running, far too much bounce!
My compromise pack for Hardmoors 55 this year (and I've made do with much worse) is (2nd best ultra pack I have) Last Drop 10 with an Inov-8 zip pocket attachment on one strap and removable zip pocket from a Nathan two-bottle waistpack on other strap (or maybe waistbelt) if I can find it. Reckon that'll about hold enough food upfront so I don't have to take pack off between CP (unless I need waterproofs etc...). The UD, UA, Nathan or bigger Salomon pack will have to wait now.
Cheers for all advice so far.