I like the 300 series, but i cant help but feel they've grown somewhat dated when you look at what the Mustangs bring to a price tag almost half the price. Its like they're showing their age, being the first wave ships, versus late 2nd wave ships.
They all have that middle room which is a disappointing waste of space as it stands, and then when you look at the Mustang Beta, it manages to fit an entire living quarter into a smaller space. Thats 18mo of refining ideas, exploring boundaries etc. Hopefully these are just early passes, like they've done with the Constellation getting a fairly significant internal remodelling and like they're talking about with the Cutlass. I've no doubt they're not 'final' but hopefully they'll be worked on rather than given a final look over and a polish.
In terms of fighters, i think the Delta will probably be better, and is about the same price (within $10 surely), and the Gladius would be a fair step up too, although theres no package for that till its flyable (which really wont be far away now), but i suspect thats $100-110, but its an ex-military ship by design, rather than a commercial ship put to military purpose.
Both the Delta and Gladius are dedicated fighters, they hold no cargo, so unless you were playing with others you'd be fairly limited to combat for earning, whereas the 325 is more of a commercial ship tuned for combat, but its still able to do other roles.
The last bit is fairly significant IMO. Multi-purpose vs Dedicated. If you're flying with others, or more specifically, you've got access to others (99% of orgs will openly share ships, certainly the small-medium ones) then it tends to be that you'd be better with a dedicated role, with everyone supporting a specific job (ie 3 fighters, 1 cargo/mining ship) and in those instances being ideal at 1 role is better than being capable at 5, good at 1.
If you're on your own, multi all the way. Try all the different roles, see what appeals, earn some money and progress from there.
The 325 is certainly a decent ship to start the game with though, you wouldnt be disappointed.
- edit -
Went to check the prices, forgot you cant get the Delta as its limited sale, and the Gladius is available (not flyable yet, soon though) at $126 inc VAT (forgot about that on those earlier prices too) Delta would have been about the same price as 325 though.