I agree, I would not have gone for anything less than 3000HH
Now, the difference between these 2 tent packages you listed are....the more expensive has a better sleeping bag and flock air beds not just PVC.
I personally would not buy either of these, as you see them around on ebay early summer, sold by people who have used them once and hated it because they had a rubbish time! There is no sewn in ground sheet (sig) so you would be likely to get creepie crawlies in the tent, the waterproof is rubbish and the layout is not practical ith kiddies.
If you are prepared to spend another £80 ish on top of the more expensive one, can I make a suggestion?
If you look at this tent here vango orchy 500 it's a really good tent that a lot of people rate, it sleeps 5 but you can always do with extra space, it has a good resale value after one use, will be more than you would get with the Argos one if you tried to sell it! This one has a sig, and has a HH of 3000. (more waterproof)
Also, just take some cheapy air beds something like these at the bottom of the page. take your duvets and pillows, make sure you also take blankets for under beds to keep out cold....I bought sleeping bags for the kids, they hated them, now they use their dvets, wasted money on those....
If you decide you like camping then you have a decent tent to keep hold of and you can just stock up on the rest when you need it, the main thing is the tent!! If you had the cheapy one, and liked camping, you would be searching for another one in no time, then you'd have the cheapy one that cost you £100 sitting around and you would resent having spent that much on something you wont use now!! (trust me, I know from experience!!)
Apologies for the long post, but I'd hope you get the right equipment to start with!
Good luck
Emma