No-one along the way seems to have discussed winch launching versus aerotowing.
Some sites do only winch launching, some only do aerotows and some do both.
Winch launches generally cost in the region of £7.50. Aerotows are in the range £25 - 35 (all depending on the club used).
Depending on the site (length of run) and weather conditions (wind direction and strength) winch launches average 1000 - 1800 feet. Aerotows are generally to 2000' with the option of going higher for a bit more money.
The OP asked the question about how long flights last. The comments about c. 8 minute flights in winter clearly refer to winch launches to 1500 - 1800 feet. 1000 feet launches = about 4 minutes! 2000 foot aerotows in still air (winter, no thermals and no ridge available) would give flights in the range 12 - 20 minutes including the tow time and depending on the exercises done in-flight.
As to the best system, initially more time in the air gives better grasp of basic flying manoeuvres but the launch and landing are the trickier bits which come later and need practice. With aerotows you tend to do fewer launches and landings (cost!).
So a club which has both launch systems might be the ideal. Before joining a club a few pertinent questions about launch systems and flight times would be advisable.
Generally 7 day week clubs = paid staff = more expense but better availability of flying. Weekend-only clubs = unpaid personnel = lower costs but a higher frustration ratio
You pays your money and takes your choice