That is a tough choice. I absolutely love my 265 Express and it would be hard to get me out of it into anything else, but if there was another Grady, it would likely be a marlin. My thoughts on both. I have fished on both and run the 265 40-50 trips a season.
265:
Highs
center helm
helm seating for more then marlin
notched transom-better for fishing around
better on fuel then a marlin
cheaper to operate
cheaper to put in a slip
big fish box in deck
best visability from center helm
lows:
low windshield-cannot cruise with eisenglass down unless you like wind in your face
center helm-some complain about it being cramped, but I have no problems or issues with it and a HATE people bumping into me so to me it isn't an issue
smaller boat-will feel more in rough seas
express style makes it harder to walk forward, especially with lee jr outriggers
30 Marlin:
High's
bigger boat-will ride better in rough seas due to added weight
more cockpit space
bigger cabin with more room for sleeping
more headroom in cabin
walkaround-easier access forward
option of a generator and A/C
low's
more $ to run-less efficient
more $ for slip
more initial cost
less interior space due to walkaround if compared to similar 30ft boats
stardboard helm seat-less visability
less helm seating for crew
eurotransom-puts you 6ft from cockpit to back of engines, tough to fish around if you venture after bigger fish
aft seating-cuts into cockpit space and is only useful underway, facing wrong way for fishing
fishbox and livewell intransom-more ate up space
Basically the 30 has the advantage in ride due to size and weight, but space wise the biggest gain is in the cabin with more space. Typically the aft bunk is used for kids or storage, know of few owners who have adults sleep there but there are a few. The cockpit of the 30 puts you at a huge disadvantage if you fish, especially offshore. good luck swinging a tuna or marlin around the outboards with stand up gear. even inshore species can be a challenge at times. The helm of the 265 is hands down better. The marlin has easier passage to the bow if you anchor a lot or go forward in rough seas. It can be fished, but only in calm seas, I've fished tons of walkarounds, they are a pain unless its calm up forward. If you might need a generator, the marlin has the option and space, the 265 would need a portable one that can only be orperated when drifting or anchored, I have a portable Honda EU2000i for mine, produces a 15A charge and burns maybe 1.5gal all night. The transom of the 265 is super easy to fish around, it is dry...I've backed mine down on fish at 5-7kts without water in the cockpit. The 30 is pushing it size wise to operate alone, the 265 is no problem. The 30 will cost more initially and to operate as well. Both are great riding boats for their size, although the 265 rides like a bigger boat in a chop.
I'm not sure what is best for your use, not sure if you plan to fish more or cruise more. Not sure if you venture offshore or stay coastal. If you fish with a big or small crew. Both will handle 4 fishermen without a problem, the Marlin could handle more if you use the walkaround and fish forward. If you need more interior space, the Marlin wins, but after a while many owners don't use it as much as they think they will, some do actually use it. If you stay near shore, a bigger boat may not benefit you if you boat with a smaller crew. If you run far distances, the 265 has more seating at the helm, and allows everyone to get out of the elements, I've had 7 under there, 5 is more comfortable though. Will you use the seats in the cockpit of the 30 Marlin or do they take up space in your mind? Do you boat alone, if so a Marlin is pushing it alone, it can be done but will wear you out, the 265 is easy to handle alone, and the center helm makes it feel smaller to handle because the visability is better then most boats, it also allows you to leave and get to the helm quicker when docking alone, and if you troll it gives you a great view of the cockpit and spread. It all comes down to layout, and what suits your needs best.
Best advice is make a list of pros and cons for your use and see if one out weighs the other, and then compare that to the financial aspect and see if those reasons still make sense, also will you have the urge to move up soon if you go smaller, or are you going bigger but settling, down the road they may be other models out that suit you better if you are.
Hope this gives you a little food for thought. Best of luck and feel free to ask any questions about either, especially the 265, I'd be glad to answer any about mine.