I cannot answer your question directly, but explaining some general considerations might help to clarify what would be a sound choosing approach.
When you say:
In my opinion it is better to use capsule since it will protect the bacteria from the acid environment in the stomach.
bear in mind that this applies to gastro-resistant capsules only:
Gastro-resistant capsules are delayed-release capsules
that are intended to resist the gastric fluid and to release
their active substance or substances in the intestinal fluid.
Usually they are prepared by filling capsules with granules
or with particles covered with a gastro-resistant coating or
in certain cases, by providing hard or soft capsules with a
gastro-resistant shell (enteric capsules). (Ph.Eur.5.0.)
Many manufacturers of probiotics use hard (gelatine) capsules which are not resistant to stomach acid. In fact these two sorts of capsules have to comply with different pharmacopoeial requirements:
Disintegration. For capsules with a gastro-resistant shell carry out the test for disintegration [...] use 0.1 M hydrochloric acid as the liquid
medium and operate the apparatus for 2 h, or other such time as may be authorised, without the discs. Examine the state of the capsules.The time of resistance to the acid medium varies according to the formulation of the capsules to be examined. It is typically 2 h to 3 h but even with authorised deviations it must not be less than 1 h. No capsule shows signs of disintegration or rupture permitting the escape of the contents. (Ph.Eur.5.0)
Whereas for hard capsules it says:
Use water R as the liquid medium. When justified and authorised, 0.1 M hydrochloric acid or artificial gastric juice R may be used as the liquid medium. [...] Operate the apparatus for 30 min, unless otherwise justified and authorised and examine the state of the capsules. The capsules comply with
the test if all 6 have disintegrated. (Ph.Eur.5.0)
Some strains of probiotics are found to be acid resistant which can be enhanced by formulation factors other than gastro-resistant coating. On the other hand, manufacturers of probiotics in gastro-resistant capsules often state (on their websites e.g.) that they deliver more units of probiotic bacteria to the intestines than conventional dosage forms. (It may depend on the strains they use.) Some probiotics can be found in yoghourt which is definitely not gastro-resistant.
Regulatory requirements are much stricter for medicines than for supplements in most countries, so if one is concerned with the sufficient drug delivery and the accuracy of medical claims, it is useful to know that for medicines to be approved for marketing much firmer evidence about these (and other concerns) has to be submitted.
Why does powder exist as a dosage form? There may be several reasons to produce and market oral powder as a dosage form: some people have difficulties to swallow capsules, it may depend on manufacturer's production line, powders allow for individualised dosage (measuring the dose for a specific patient - but I don't think this would be necessary with probiotics), hygroscopic excipients which are incompatible with capsule shell etc.
The dose you take in powder is not more precise - even if it is sold in divided doses (each dose in one bag), because a small amount of powder can always remain on the walls. The precision of measurement is the same at best, if not in favour of the capsules.
How to chose?
Here are some of the questions that should be taken into account:
- Is the patient allergic/intolerant to any of the formulation's ingredients?
- Is there a form preferred by the patient's physician, for some medical reason?
- Which form is the most convenient one for you?
- Which form is accessible and affordable to you?