Microneedling & PRP: which method of applying PRP Is best? Via needle or smearing it on the face?

For optimal, multi-layer results it is best to both smear and inject the PRP after microneedling. Enhancing full thickness collagen production is the main objective which in turn gives the best results in collagen strengthening and textural improvements to the skin. These procedures are best provided by licensed Vampire physicians that are specifically trained in full thickness collagen techniques. As a side note, nonmedical personnel frequently do not inject PRP under the skin. Limiting the treatment to smearing does not provide the same quality results.

The answer to your question depends on the amount of correction you need and the results you hope to achieve. In my office, I offer the Vampire Facial, which involves topical application of the PRP prior to microneedling the area. The micro-needles allow the platelets to penetrate into the lower layers of dermis where they can produce a positive regenerative effect. 
My Vampire Facelift combines topical application of PRP with injections that go deeper into the dermal structure. The depth of this treatment stimulates even more collagen production, improving the skin at a cellular level. Because patients tend to see more rejuvenation with this approach, it is called a facelift as opposed to a facial. I encourage you to work with a board-certified plastic surgeon with expertise in PRP treatments. This doctor will be able to tailor your treatment to your precise needs and expectations for the best possible outcome.  

As with many of the procedures utilizing PRP, there is no absolute standard of care. Many will apply PRP topically allowing for absorption via portals created by microneedling while others will inject and still others will employ a combination of both techniques. Ultimately, the goal is to deliver growth factors to the site of injury/inflammation promoting rejuvenation.
Without, good randomized data, it remains unclear which approach is best. It is likely that the difference between the approaches is negligible as long as the PRP is delivered where it is needed most (either topically or via direct injection or both).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
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Delivering  PRP to the skin to the depth of 1 to 1.5 cm is ideal. This can be accomplished with a needling device and cartridge containing PRP.Jeff Rapaport MD FAADRealself Verified All-star

It depends what type of improvement you're looking for. Each method of treatment is for a specific type of concern. Injecting PRP will help fill lines and indentations, much like filler while micro-needling will help with skin texture and quality. 

The two most common ways to use PRP on the face are to place it topically in combination with micro-needling or to inject it into the face.  Placing topically is often call the vampire facial and injecting is often called the vampire facelift.  Placing topically enhances the micro-needling results and will have a more uniform affect on the face.   Injecting is more targeted treatment and is often used like a filler or in combination with a filler.  For example, you can inject the eye through, nasal labial folds,  the cheek  or the lips with PRP to enhance that part of the face.  

What  I suggest is a combination of prp injections followed or preceded by Microneedling with ppp. (platelet poor plasma)

See the recent post in the web-reference below which I wrote on my forum addressing PRP vs Microneedling by a review of the literature at the recent International Hair Restoration Society meeting