The first query may not be doing what you expect. If you reference a column from a left-joined table in the WHERE clause, the left join is converted to an INNER join - except for the special case WHERE [column] IS NULL.
To preserve the outer join, change your query so that the filter is applied to the join, like this:
UPDATE a
SET [omschrijving] = SP.[omschrijving]
,[verkoopprijs] = SP.[verkoopprijs]
,[gewijzigd] = getDate()
FROM [artikelen] a
LEFT OUTER JOIN [Hofstede].[dbo].[sparepartsupdate] SP
ON a.[PartNrFabrikant] = SP.[PartNrFabrikant]
AND ((A.omschrijving <> SP.[omschrijving]) OR (A.[verkoopprijs] <> SP.[verkoopprijs]));
Note that for rows in artikelen which don't have a match in SP, [omschrijving] and [verkoopprijs] will be set to NULL - is this what you are expecting?
Is PartNrFabrikant a primary key in either table?
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden