Options
From an ATC perspective a vertical separation is easy to apply, it immediately demonstrates safety, allows the aircraft to fly in a straight line, and is economical on R/T.
Radar vectors need monitoring, both aircraft need to be controlled and subsequently de-restricted, the heading[s] may cause infringement of neighbouring sector requiring co-ordination, possibility of tracking outside permitted corridor/airway with proximity of Danger/Restricted/Military airspace, and of course the use of radar vectors on multiple traffic elements will adversely affect sector capacity.
Over to you pilots......what do you want.....to fly in a straight line or maintain the level and add maybe 20 track miles?
Pointer....hold out your arm at full stretch and look at the width of the fingernail on your index finger....that is 10 miles on my radar......and my ATCO colleagues and I will never risk your safety on that........and avoiding wake vortex could have been a factor.