We have a 5m bell tent and you certainly get loads of space, although obviously, the sides are sloping so you don't perhaps get as much space to walk in as you would with a really big tunnel.
Although bell tents are heavy (20 - 25 kilos I think), that is not dissimilar to many large family tents, at least polycotton ones. Provided you are car camping, it is probably not too mcuh of an issue.
What you lose in a bell tent is windows - there are little mesh ones round the bottom but you can't sit inside the bell in the rain and look out. Not an issue for us as we would have a tarp/gazebo or go out in the rain, but it is worth thinking about.
Cost-wise bell tents are pricey if you buy them in this country. You can get them cheaper from Obelink in Holland but they won't deliver to the UK (restriuctive agreement with Belltent.co.uk - lots about it on here). If you can take a trip to the Netherlands, it is worth doing or getting them delivered to an address in Europe, if that is an option. Otherwise, Soulpad is fairly reasonable I think. Or they come up second hand on Ebay occasionally but they do hold their value quite well.
For 6 people, you would be better going for a 5m rather than a 4m bell. If you can bear having a separate groundsheet, that would make it both lighter in weight and less expensive. However, some people are concerned about bugs etc and prefer to have either a sewn in groundsheet (SIG) or, a zipped in groundsheet (ZIG). THe last option is the priciest but allows you to unzip and roll up the tent sides on a hot day and also you can pack the tent and groundsheet separately which is useful if it is muddy.
I think if you want a bell tent you will be unsatisfied with anything else but if you are fairly unbothered there are lots of other tents which are probably more practical for a family.
As we have a bell and a family pop-up from Decathlon (which I think would be too small for 6 people) I won't comment on other options as there are lots of people on here with much more family tent knowledge than I have.
Have fun choosing 