Price

Polywrapping will typically always be less expensive than the equivalent envelope enclosing solution.
There are more Royal Mail services where you can achieve greater postage discounts when using envelopes over polywrap.
The weight of an envelope compared to the equivalent polywrap can be a few grams heavier. This can be especially important if trying to keep the overall weight of the mailer below a postage price threshold.
As polywrapping uses a roll of film that is wrapped around the mail-piece, it is a lot easier and therefore cheaper to wrap non-standard sizes of products. Compare this to using envelopes which would require expensive bespoke envelopes to be manufactured or the use of envelopes which are too large which can look clumsy as well as damage the envelope during delivery.

Environmental Considerations

Use of recycled envelopes works out considerably more expensive than envelopes made out of non-recycled paper.
Recycled polythene is not commercially available/feasible. The environmental compromise is polythene that is biodegradable which significantly reduces the time to degrade if placed on landfill. There is also a difference in price between standard polythene and biodegradable polythene, however this is not as significant as the difference between the envelope options.
There can be issues with recycling envelopes as some local authorities require windows to be removed.

Protection of Mail Piece

There is greater scope for visible damage to envelopes in terms of getting crushed or folded as the mailer makes it way through the post.
For heavier items, envelopes are typically less durable than polywrapping and more likely to suffer damage as the mail is handled.
Polywrap is waterproof compared to paper envelopes, which can prevent damage during delivery

High Impact

With polywrap you have the benefit of having a high impact product that is immediately on display without the recipient having to open the item.
Following on from the above, it is conceivable that more people at the recipient’s address will be able to view the mail-piece. This is especially true in a business as the mail can be passed from mailroom to reception to the recipient.
However, achieving a high impact may not always be desirable. Take the sending of sensitive material such as bank statements or payslips, but can also include literature which some recipients may want to keep private e.g. pregnancy or political brochures, in all these cases the use of polywrap may not be a suitable solution.

Other Considerations

Polywrapping may not be suitable/cost-effective for small mailing quantities.
Polywrapping can give a more professional appearance as this would need to have been outsourced to a mailing house.
Both polythene and paper are volatile commodities, so where one option may be the lowest price on one occasion may not be the same in future.