In Jane Magrath's standard grading reference The Pianist's Guide to Standard Teaching Literature, the Scarlatti "Aria" is Grade 7, and the Bach C Minor Prelude is not included, placing it beyond grade 10.
Based on my own experience, the Scarlatti is the markedly easier of the two. Its slow tempo allows "breathing room/thinking space" for the performer to overcome difficulties. There are lots of leaps, but once made, the fingering is mostly straightforward. The piece is also relatively short (24 measures) with clearly delineated phrases. For a new pianist, the leaps, ornaments, and hand coordination would be primary issues; the overall musicality requires a great deal of maturity.
The Bach requires fast and much more intricate finger-work, with a faster harmonic rhythm (the pace at which the underlying chords change), and although the fingers play simultaneously, they often play different patterns, which can be challenging to coordinate. The piece overall is longer (38 measures), and much more complex, with more ambiguous phrasing.
I agree with your initial judgement that these two pieces are not at the same level. However, I would still encourage you to try the Bach, since you're motivated to do so. If it's genuinely too difficult, you'll recognize that, and there are plenty of other pieces available to help work up to it. Perhaps despite any difficulty, you'll still find it satisfying to play at a slow tempo. Or you'll find yourself motivated to work through the piece. Or it will prove not so difficult for you.