If you consider the total time after passing class and ten, both the rout would take the same time. After passing the tenth Examinatin and then persuing the Diploma in ECE, time consumed in such a mode will be three years and later taking up a course of BE in ECE would take additional three years since you would be admitted in the second year of Engineering subject to clearance the written and oral test set for further perusal of BE. Consider the later option where you spend two years for 10+2 and later four years for the Engineering - degree completion in ECE. Time - span for both the cases is the same.
Whatever, the mode is selected to achieve the end result, you should be dedicated to your mission and should stay focussed in your studies so as to get the best result.
However, I would select the later part ie an option to pass 10+2 first and later to join a Premiur Engineering college of four year duration to achieve BE in ECE. Such an approach would not create any ambiguity later with respect to any change in terms of mode. But in case in opting a three year diploma and later to join an Engineering - college of four year duration may not materlise in the event of failure of the written test being followed for admission. In that case, you will end up with a certification of Diploma only.
As regard to your last query, you may choose either of the option leading to achieving your ultimate goal.