A basement shelter frees up option / space in your 1st storey for other purposes (ie: space planning for living room, dinning room, kitchen, etc). Not to mention the asthetacical appeal benefits as some people may feel the steel door or the steel vent opening unsightly. You also don't have to based your interior design around the shelter's unsightly doors, vent, etc
If you have the budget, I strongly encourage it.
You can have one of the shelter's walls to double up as part of basement wall.
You should be aware of the requirement by Civil Defense on the internal floor clearance area & the volume in relation to type of housing / occupant's number.
Google the "Civil Defense bomb shelter handbook" & download it for additional information.