Neither, both, take your pick. There was no uniform opinion among German fighter pilots as to which machine was the more formidable enemy. There were also no polls among german fighter pilots in general so we don't exactly have a big source of data. What we do have are the opinions of the German aces who test flew captured allied planes at Rechlin test center. Namely Werner Mölders, Walter Wolfrum and Hans-Werner Lerche.

The emphasis of these test were generally not to learn things which the German aviation industry could take and implement in German designs but rather to learn the strengths and weaknesses of the allied fighters so that this information could be passed on to German front line pilots. As such the focus was not to find out which was the better plane, rather to find out what to watch out for with each plane and how to shoot it down.

Overall, the German test pilots had favourable opinions of both the Spitfires and Mustangs and generally speaking found them comparable fighters to the BF109s and FW190s with different strengths and weaknesses.

Lerche actually published a book after the war, it's called Luftwaffe Test Pilot: Flying Captured Allied Aircraft of World War 2, if you want first hand accounts of what he and other aces thought of the caputred allied aircraft they flew I recommend it.