I’ve currently got a Lotus and a Guru with a range of ICUs (Small, medium, large). Being able to swap ICUs in and out is the best part. Since I’ve started to downsize kit I’ve found I’m using the Guru the most as it fits a 100-400 with A7r3 attached with room for a second lens (70-200 f4 or 24-105 f4) in a small icu and still have room to spare above the ICU. For most wildlife trips that’s all I need. It can fit a medium icu in but I find it’s easier to carry the medium in the Lotus. In a medium ICU I can get 3 lenses (100-400, 70-200 f4, 24-105 f4) with one attached to the camera. There is room in the ICU to be able to swap over each lens ie the 100-400 and 70-200 can fit in the side sections with the 24-105 is attached the the camera in the middle section.
If I’m off out for landscapes then it’s the Lotus I take as it has more room in the top. The harness feels better too. It can fit a large ICU in but I don’t use that much now as I find my kit fits quite nicely in the smaller ICUs. The best harnesses are on the larger Tilopa and Sukha bags. They seem to be much more comfortable than the smaller bags in you are packing them to the brim. If you are thinking of the Ajna I would consider the Tilopa for the better harness especially if it was ever going to be pack full of heavy kit (camera, other bits and water) then the better harness could come in handy. Between the bags it all depends on the amount of spare space you want.
The guru bag I have is more along the same lines as the loka ul (same bag series). The materials are noticeably thinner than the mountain series bags but I like the side mesh pockets on the guru. It’s easy to put a bottle or tripod in them and you aren’t restricted by the size of the side pocket openings like the mountain bags. The side pockets on the mountain bags are a great addition but they could be better. The side pockets are just empty pockets. There are no organiser pockets inside on the bag side which is a shame as everything you put in them falls to the bottom of the pocket. You soon learn they are best for stuffing in gloves and hat over a water bottle. The side pockets also have a Velcro tab that’s supposed to allow the pocket to expand slightly but it’s annoying opening and closing the velco so I’ve positioned so the velco its only closed at one end rather than the whole length. The things I like about the guru style bags are the mesh side pockets and the hip belt pockets. The hip belt pockets are good for putting the lens cap or a cereal bar in. The side mesh pockets can fit a water bottle, a travel tripod in or two legs of a bigger tripod (there is a side strap further up that secures it nicely to the side of the bag.
The front pockets are great for stuffing in a thin jacket, fleece or insulating jacket. They are really useful for quick access to an extra layer.
Over the last few months I’ve been impressed with the guru and it’s design (I like the organiser section that’s between the front pocket and the main compartment. I don’t think any other bag has this feature. I’m often using the guru as a normal bag especially for taking my laptop to work or down a coffee shop whilst the wife is at the hairdressers). I had a thought of swapping the lotus for the Lola ul because im finding the mesh pockets and hip belt pockets are very useful. But it feels like I'm probably better to stay with the guru and lotus as I have a bag from each series and the benefits from both. I can live with the positives and negatives of each.
@jerry12953 is right that things can fall down the side of the ICU. I try to mitigate this by using an organiser pouch inside above the ICU or put in things like filter pouch and holder pouch in this section (ie not really small stuff).
These are the best info guides I’ve found on the net:
http://webshed.co.za/test/2016/07/11/choosing-the-right-fstop-gear/
http://webshed.co.za/test/2016/07/11/choosing-the-right-f-stop-gear-2/
Places to buy F stop gear from in the UK are pretty much no existent. The best place I’ve found is Alpine Trek. They are based in Europe but delivery was free over £50 and was a couple of days when I last used them.