Thameslink is shit in my experience, but it's still not better than the Tube to get to Heathrow. My point was more that it's 70 km and one hour's drive further away at 70 mph for a lot of people.
If you want to talk about convenience and cost, Heathrow hands down. For our family of three:
Tube off-peak to Heathrow: £4.60 (£9.20 round trip)
Tube peak to Heathrow: £6.40 (£12.80 round trip)
Tube and rail off-peak to Gatwick (contactless): £24 (£48 round trip)
Tube and rail peak to Gatwick: £37.25 (£74.50 round trip)
Local taxi firm (15-20% cheaper than Uber and more reliable):
Heathrow: £27.30 (£54.60 roundtrip)
Uber (25% cheaper than local taxi co.):
Gatwick: £72 (£144 roundtrip)
Rail to Heathrow is 40-60 mins and 60-80 mins for Gatwick. Arrive at Gatwick on a Sunday evening and the line to Clapham Junction is closed for engineering works, then 150+ mins on a train that goes out past Crawley and loops through Surrey and back in (yes this happened to us). Better, cheaper and quicker options from Heathrow when things go wrong.
Heathrow annoys me because there's so much retail obstacle course after security (this is problem in many airports these days) and they've sacrified a lot of seating for it. Last time I was at Gatwick, I couldn't find any water fountains after security, and lack of free drinking water prejudices me against any airport.
BTW, I don't use the Lizzie line to get to Heathrow as that would require going out of my way to Ealing Broadway. It's District and Piccadilly Lines all the way. I realise experiences differ depending on where you come from. I know you're somewhere south London, and I'm SW near the South Circular and District Line. But, Gatwick just doesn't stack up for me and the cheaper airlines rarely work much cheaper once baggage and additional ground transportation costs are factored in, but they do tend to be more aggravating and their destinations less convenient or they're less resilient when a flight is delayed or cancelled.