This is based on my own experience dating back to my teens when I first tried an open-top
Hot Water Bottle Style
Pros:
Obviously a HWB-type is neater than an open-top.
Nothing spills if you have a good valve.
It's safer. Nothing gets added after it is sealed. you KNOW what is in the bag when you seal it, and that's what you are taking, at least for this round!
The water stays at temperature longer in an HWB style bag.
They seem to be easier to find in serviceable condition.
Easier to mix. Since warm water is best you end up with cold soapy water, adding hot water to it to get it warm.
You can turn the bag over several times and even let it lie on it's side through the final stages of prep.
Cons:
Harder to clean
Once cleaned takes longer to dry
Seems not to get the serious klismos as aroused as quickly.
The last pint or so takes longer than the rest because of back pressure.
A little more waste A couple of ounces stay in the bag.
Open-top
Pros
The more Klismo you are the more arousing it is seems to work faster than an HWB style
Easier to clean.
Steadier, more even flow
Makes a sucking sound to indicate when it's empty. Easier in the knee-chest position because you don't have to look at it to know.
No waste. No water stays in the bag.
Cons
Messy! Almost impossible to avoid some spillage, especially in the later rounds when you can already be a little discombobulated.
If the room is not secure, substances you DO NOT want inside you can be slipped in by a well-meaning but dangerous SO, or sibling
Just more awkward to handle
Water cools faster!
That said, I have always preferred HWB style for most enemas.
My 4-qt bag is an open-top.
I have used a 2-qt open-top for my worst, "rough ride " enema