I used to own an Asp and a Vulture before I went on the Distant World's expedition and when I came back I decided to upgrade the Vulture so I bought an FDL.
This required a completely different style of combat though. The Vulture is far more manouevrable but slower. The FDL is much faster but I found it more difficult getting as close to the enemy and it takes forever for the shields to come up, so I need to use SCBs. But if I can get the hang of controlling the speed then it will be a beast. I ended up missing my Vulture a lot after I sold it so I bought a new one. I was also told that the FAS is an upgrade to the Vulture and being manouevrable as well I thought I would give that a try, so I bought that to compare against the FDL. It's definitely a lot of fun and in a sense it is a direct upgrade.
I'm trying to improve my combat skills, so like with learning a range of different martial arts, I have bought a range of ships that work best in different scenarios. I then bought a Viper Mk3. Next will be a DiamondBack Scout because I had such good times with that ship in the past and this could be good for stealthy combat. The idea is that each ship promotes a different play style and if I can get good at all of them then it will mean that I can cope with whatever situation I come across.
But here's the thing, for me personally given my current play style, I actually do best with the Vulture. But this only really clicked when I replaced the C3 cannon with a frag cannon. I combine this with a beam laser. Yesterday it really came together and I was chewing up NPCs 1v1 in a HazRes faster than I have ever done before. I'd beam the Asp/DBS/Adder, then three quick frags in succession and they'd be down to 30-40% hull integrity. It was a real blast and out of all my combat ships I have most fun in the Vulture. I went 1v1 against a dangerous Anaconda and got him down to about 8% when he blew out my canopy when I wasn't close enough. I only had 5 minutes air but I continued anyway, destroyed him and got to the station within 2 minutes. I really wished I had recorded that.
So what I have learnt is that it all depends on how you fight. This determines which weapons work for you and which ship is best suited to that load out. It also depends on what you want to use it for.
I'd pick the Vulture for taking down big ships because it's so small, manouevrable and can get in close behind or at the side and stay there with lots of chaff so most of the turreted weapons are useless against it. For farming a HazRes or CZ I'd pick the FAS. Or if I was protecting an unarmed explorer or trader so I could ram any pirate as quickly as possible so the other player could escape. For PvP I'd choose the FDL which might outrun the Vulture, but it would be interesting to see how the Vulture coped. If I wanted to act as bait then I would choose the Viper.
If I could only choose one, right now, for me and the way I personally fly, it would be the Vulture. When I get better at controlling the speed of the FDL, making better use of the huge plasma accelerator and possibly adding another fire group to it, then my opinion might change.