If you get both done at once, you have no objective way to independently compare whether the results are better than what you already have....
My doctor was pushing me to get both done at once, but I wanted to have a control to compare against, and boy am I glad I did.
Both my eyes were about -6, but I only had my right eye done (left eye not). It is useful having one eye that doesn't need contacts. But it is also useful having one -6 eye because it makes close-up work easy (I can't use the right Lasik'ed eye for fine detailed work - it can't focus on anything closer than 30cm - like long sighted but actually isn't).
With a contact in my left eye, and the Lasiked right eye, the left eye has much better clarity even in day time. The results on the right eye were normal (this situation isn't because things were stuffed up).
The halo effect is due to uneveness/dimples/ripples created, so can never be corrected by lenses.
Anyone that has both eyes done at one time cannot make an objective comparison of advantages/disadvantages.