hi Paul, good question, the brands without the name tag on maybe good for about 2 or 3 hits, but normally the handle breaks, they may look the same but in actual fact it's not the same timber.
I wouldn't go and buy a screwdriver set from K-Mart or at a $2 shop, they are cheap, but useless, the tips don't hold their shape, they break at the handle, and especially if a screw is screwed in so tight they will do more damage than Mandrake, so you are wasting your money.
If you are going to use that area as a BBQ with a table and chairs then I would use the 2400 Watt, especially as it's got a 3 year warranty, however if it's just to highlight your work then save electricity and use the 1600 Watts.
A long crowbar or digging bar which is long and made out of steel will come in handy also to break up concrete rocks, you can hire these.
You can also hire a 'jack-hammer' for half a day, if you feel it will quicker and less energy, but having a sledge hammer may come in handy another time.
Digging through clay is b*****y hard work, but you can always leave a tap dripping where you want to dig, it will be softer but messy, there is also a post hole digger, manual or petrol operated.
Before you want to dig a hole in hard soil leave the tap on, just dripping, it will also soften the soil.
Use 'rapid set' concrete when the holes have been dug, the concrete 'goes off' much quicker.
Good luck but hard work. Geoff.