Bro ,cultus is good in all aspects you mentioned, it's a good economical car... If you don't need comfort and satisfied with cultus CNG economical drive then dont change it.. it's my personal view..
Cultus, as said by showroom guys,is CASH..there are MANY ppl who would give u instant cash for cultus in good condition....
Regarding passenger compartment
for vitz it's
The size of the passenger compartment (LxWxH), mm 1855 x 1390 x 12
The size of the passenger compartment (LxWxH), mm 1920 x 1350 x 12
for alto (japanese)
The size of the passenger compartment (LxWxH), mm 1835 x 1260 x 12
see for alto it's small but MIRA and VITZ almost same...and since vitz ka agar aap ne check kia ho to mera personal khyal hai k MIRA ka definitely cultus k brabar hoga ya little bit small( correct me if i'm wrong)