There's pros and cons for both - though just wondering why you narrowed down to the two you did?
No quibbles with Marantz from me; great brand, great pedigree, great sound, but I like that presentation. Of the others, there's a bunch up to £250 you can choose from. I'd also add the Harman Kardon HK980 amplifier. If you can forgive thae, frankly, daft volume knob (cosmetic flim flam that lights up when the amp's on to give a kind of moonglow effect, or something), I heard the much bigger (physically and design-wise) HK990 and can only report good things. If the 980 is half the amp, for £250 you won't go wrong.
If you opt for the used route, there's the usual pros and cons - better kit for less money, as JD mentioned, you'd be hard pressed to clap hands on a new Primare amp for much less than a £1k sum, never mind a whisker over £300.
My tips - Technics SU-V6: hugely underrated, massive build, very very clean sounding amp and very easy sound to live with.
NAD - C370 or the earlier 319 which both picked up good reviews, especially the former (I think Stereophile still have an online one you can check out).
Lastly, there's my one which is simply the best amp I've heard - Sansui's AU-717. Internal build to die for. Mine cost me £200 delivered, allow £75-100 for a good service to replace corroded capacitors (mine had a load of smaller ones needing replacing, but it still sounded good beforehand!) and you're in for £300 or so. Difficulty is i) finding one and ii) accepting - as you have to with any older bit of kit - the need for a bit more TLC. Once all that's done though, you've got a class leader.
I tried out a Harman Kardon HK990 and a Leema Pulse a few weeks ago while my 717 was in for repair, had the Pulse at home for a week and though both were very good, excellent in fact, when the Sansui came back and was now fully fixed, there was only one winner. The effortless ease that comes with any music you play with one of these is hard to describe, you just forget there's any gear there at all and enjoy the music, but enjoy it in a way that makes it sound more real and more lifelike.
Whatever you decide on though, enjoy it and happy hunting!