on some CNG balenos, it do backfires,
but its simple solution is to properly tune ur car gas/air/CO etc.
i had kept baleno 2001 for about 4 years,
Ma sha ALLAH not even a single backfire in 4 yrs,
and now i have 2005 baleno, its also awesome, no backfire problem.
go for baleno, as margalla is very old car, (at max 1998 model)
and brother remember, its very difficult to maintain a 15 year old car, jitni chahe saaf ho, but tang bohat karti hein itni purani gaariyan.