Good question and one that comes up a lot. Increase power vs decrease weight - it is a balancing act for sure, and there is no one answer that applies to us all.
Generally, if you are a bigger rider, who is ‘over weight’ and has ‘plenty of timber to shed’ (a BMI of around 30) then you are going to see the greatest gains in speed and w/kg, by loosing weight. Maintain your fitness and power, but reducing your weight.
Conversely, if you have a BMI of 20 (so pretty low), not only are you going to struggle to lose weight (as you don’t really have it to lose), but also it would be unhealthy to do so. In this case, you would be far better trying to increase your power through training.
Looking at your position David (5ft 10" and 75kg) - you have a BMI of 23.6 - so whilst you have a little weight to lose, there isn’t any ‘low hanging fruit’. So you are the type of rider who I would argue is best to try and do both; lose a little weight, and focus on developing your power. You need to fuel well to increase your power, vs maybe some fasted riding and a calorie deficit to lose weight, so a balancing act. Your 240W FTP is so personal, it’s not possible to comment on whether you have plenty of progress to make or whether you are nearing your max, without looking at your training history. Give some ‘Over - Under’s’ a try - a great session for nudging your FTP up.
Will cover your whole question, inc. the weight training aspect on the Podcast. Kev will have plenty to add specifically on weight training, but check out Episode 23 - Track World Champs, where we talk about weight training a little towards the end.
Top question and thank you,