You asked "How does one decide?". There isn't a simple answer to that, especially for a novice making their own choice.
Those of us in the trade who want to do it right spend time studying and use our field experience to validate what we learned. That helps build up some local knowledge of what aerials will and won't work in certain areas, and it also helps to sort the wheat from the chaff with all the aerial choices.
The crunch point for me came after a couple of years running my own home cinema & audio visual installation business. Aerial installs was something I normally handed off to local installers, but often I'd find that corners were being cut that either affected the quality of my customer's signal or that made my job harder. That was when I decided enough was enough. I did some training, lots of reading, bought some proper test gear and really got to understand what the differences are between a good and a poor installation.
That SLX aerial claims to be a 36 element rig. That's BS. They're counting the cross elements as 4 each when they're really just two at most, and counting other bits as multiples too. It's more like a 13 element aerial, but 36 looks a better number when someone who doesn't know any different is leafing through a catalogue. From experience I can see that the general construction of the elements uses thin bits of aluminium bent in to shapes that attempt to give then some strength. But you've only got to look at a few of your neighbours houses to see similar aerials where the fins have been bent by birds and then the aerial gets mangled by the weather.
The next place people get ripped off is with cable. Good coax isn't expensive - we're talking a few tens of pence per metre, - so the cheap stuff isn't really that much cheaper yet it loses a load more signal, and does a crap job of keeping out interference, and the outer jacket weathers far quicker so it ends up causing a load of hassle with lost signal or letting water in to the back of electronics so you have to spend on the decent stuff and just hope that your telly or PVR isn't corroded to hell. That kit includes 10m of cable, and it's likely to be the cheap stuff. The difference in price netts out at a saving of £2-£3, that's all.
Your recent aerial install on the roof is a short Log Periodic. Mounting up high makes a big difference to the amount of signal that an aerial will generate. The same aerial mounted just 3m from ground level might generate as little as 20% of the signal strength just from being closer to ground level. Power in itself isn't so important as quality. Background interference is always present; so if the aerial is generating less signal then the margin between the signal and the background noise will be reduced too. You can't regain that with amplification because the amp boosts the noise too and adds its own. This makes the signal to noise ration worse, not better. A larger aerial (a bigger Log Periodic aerial) can help redress the issue. If you have space to fit a full sized Log Periodic then do it.