I'm in the process of building an outdoor fireplace. I toyed with the idea of a fitpit and it would be relatively simple to construct in comparison but I opted against it in the end.
The benefits of the fire pit is that it's more camp-fire-like so you can all sit around it and toast your marshmallows. I wouldn't construct it into a table though. You can use a table for many other things - sitting with a glass of something cool on a summers day, reading a paper or just sitting at it in the garden to get some fresh air. A specialised BBQ table will be used every so often for a BBQ, maybe every weekend for the first month after buying/building it but then the clean up will end up getting left and then you want to sit at it with you nice cool drink with rusty grates and a load of charcoal blowing up ash in your face on every small breeze. If it's like my house (I have 5 kids) we'd all sit round it, I'd spend £30 on charcoal to heat everyones little grill in from of them, then they'd all stick one sausage on their little grill and maybe a burger then they'd be done. Plus, they have to get up to go to another table that had all the bread buns and sauces on it because you can't stick them in the middle of this table without them melting into the coals.
Nope! Keep them separate and enjoy them both for what they're intended for. You can always get a grill for the fire pit as well as building a raised pit if you wanted with either fixed built in seats around it or whatever comfy seats you have to hand.
