Both tracks are majestic, breathtaking and full of history.
Spa on a rainy day and seeing the clouds hanging lower from where we were standing further down the track, standing at the top of Eau Rouge looking down and seeing how steep it is and seeing the cars take it flat out, La Source, the bus stop (now sadly gone though.....big mistake) and sitting/perching on the rocks above it with other fans, hearing the sound of F1 engines through the trees, the frites with mayonaise
Monza with all its history and many memories and memorials, the old oval looming ominously in the background (and, ahem, walking/climbing it under the weary eye of a bored guard), the tifosi, hearing the sound of F1 engines through the trees, Lesmos, Ascari, Parabolica.
Choosing between Spa and Monza is just impossible and ideally you should do both: one this year and next year perhaps?