According to my CMP index, China should currently outclass India by a factor of more than three (figures for after 2015 are based on projections but this index don’t change cardinally over the course of a few years). That’s almost comparable to the fivefold differential between Pakistan and India.
Both countries are effectively islands relative to each other, so scope for a traditional land war is limited. In the Himalayas, it will come down to a question of troop quality and airlift capacity. China should have the edge in the former – even back in 1962, it won its engagement in Ladakh against India, while incurring twice fewer casualties. Its superiority in airlift capacity might be undercut by India’s easier logistics.
I am not sure of the logic behind the Chinese belligerence.
I suppose performing some calibrated tests on the PLA’s combat effectiveness on a weaker military might be a good idea for estimating its expected performance over, say, Taiwan. The conflict with India can be dialed up and down, whereas for obvious reasons, any invasion of Taiwan will be an On/Off affair. This is an increasingly pressing issue, given the accelerating Great Bifurcation and the louder and louder voices clamoring for independence in Taiwan.
Obviously, it will sour India’s relations with China and move it even closer to the Americans. On the other hand, the Strait of Malacca is getting cut off either which way come the Next Great War, so a (more) hostile India would not be too relevant.
It does put Russia into an uncomfortable position, which has excellent relations with both China and India (in 1971, the Soviet Navy helped deter the Americans from intervening against India in support of Pakistan – and, indirectly, ensured Bangladeshi independence). I suppose China wagers that Russia doesn’t view India as the more indispensable partner, and they are surely correct to do so. Certainly Russia is not going to embargo/sanction China on account of India.