Luton is more of a low-cost, no frills hub and is much smaller than Gatwick. I would go with Luton but see which airport has the cheapest fares. There are Gatwick trains (run by First Capital Connect) that take you to St Pancras as well - you don't have to go to Victoria.

Whether Luton (LTN) is "better" than Gatwick (LGW) depends entirely on your budget and destination, though Gatwick is generally considered the superior airport in terms of facilities and connectivity. Gatwick is the UK’s second-largest airport; it has two terminals, a vast array of high-end dining and shopping, and excellent "Gatwick Express" train links to central London (30 mins). It handles both low-cost carriers (EasyJet) and major international airlines. Luton, by contrast, is a dedicated low-cost hub (Ryanair, Wizz Air) and is much smaller and more "functional." While it recently opened the "Luton DART" shuttle to connect the station to the terminal, it has historically been ranked lower for passenger experience and can feel more crowded. If you find a significantly cheaper flight and live in North London, Luton is a practical choice. However, for a more "standard" international airport experience with better lounges and more flight options, Gatwick is almost universally preferred by seasoned travelers.