I, too, own both a Seagull and a Martin. Each are down-line from the ones you are considering, but I think my experience can benefit you.
First, what I have:
Seagull S6 Dreadnaught, and S6-folk. Both bought used, paid $150 for each, altho you should expect to pay about $200.
Martin SPD-18. Bought new, paid (as I recall) $800, got a discount because it had a cosmetic flaw, but still carries lifetime warranty.
Both marques carry a lifetime warranty to the original buyer. I am happy to buy used, but that warranty is nothing to sneeze at. I can tell you from personal experience, Martin stands behind their products. I have gotten excellent support from Seagull, too, but I don't qualify for the warranty, so I can't comment on that.
If the Martin was really that dreamy to you, you will be disapointed if you buy the Seagull- it will never seem to measure up to your memories (which may be false, but will still be your memoires) of the Martin. Turst me on this one- I am STILL kicking myself for passing on that '68 Gibson LesPaul Lefty in Seattle- coudda had it for $500- FROM A VINTAGE GUITAR SHOP!
There are very, VERY few guitars that are the value Seagulls are. Yes, a martin may sell for more (perhaps even a greater precentage of it's new price) but I sense you are buying for your use, not to resell. Frankly, I am not sure a Martin really IS a better guitar than a 'Gull, if comparing apples-apples in quality, appointments, etc.
I have been impressed with Seagulls ever since I found one for $70(!) at a garage sale- never heard of the brand at that time, and felt like I was taking a risk. If I was, it paid off. Martins carry considerable cache', but I think the cost is just a little out of proportion compared to that cache' and the quality. Put another way: you will get better value with a Seagull, than a Martin.
I would bet you could find a Seagull that plays and sounds at least as good as that Martin, and save yourself about a grand. OR, contact LaPatrie, tell 'em you are considering a Martin that cost twice as much as the Seagull you are looking at, and ask them what they can do. I would bet you dollars to donuts they can build you a KILLER Seagull guitar, as custom as you want, for two grand, that will blow that Martin out of the water.
Heck, ask if they can indorse you!
Oh, ONE MORE THING; The Seagull Folk-sized guitar is better balanced (sound) than the Dreadnaught. I second the comment on buying a smaller guitar, esp. if recording.