you would vote for isb number because you live there.
let me share some incidents with you.
i got my civic ferio regd in islamabad. the copy was to be issued after verification frm customs deptt lhr. it took 2 months for excise to write a letter to them. i took that letter personally and got it verified and gave back to excise. after 1 week i asked for regd book and they said that the verification/noc frm customs was still pending (they lost it). i did the same process again and then my car's file was lost. i had to use some extra sources to get the search done on priority. still, it took 4 months for the process to finish.
got a civic regd there. it took 5 months for the regd book to be issued.
got a corolla registered in Lahore. got my book, file everything within 20 days.
i would like to finish the pain once for all and have all the documents in my custody. You cant even imagine what happens with car's files when they are left with exicse department. Especially with the docs of luxury cars/jeeps.
the only charm isb number car has is the low token tax. that is all.
as far as the resale is concerned, a well maintained lahore registered car sells equally well coz there are many people even in isb who belong to parts of punjab where lhr number is preferred rather than an isb regd car.