My impression is that they are completely different. I haven't been to the New West River Market since they have been there but I think it is a little more approachable. I can speak to having an amazing meal 3 weeks ago at maenam. My lady and I did the chefs tasting menu and it was amazing. Every dish was very well balanced, meat and seafood cooked perfectly and service was great. We had standout salmon salad with cilantro and crispy shallots that was dressed with perfectly. I am keen to try out Longtail however I am rarely out that way.
It depends. Are you looking for a gourmet food court experience or refined fine dining with table service?
Coincidentally, I dined at Longtail on Sunday - my first time back to New Westminster in 4 years. I didn't even know of Longtail beforehand, but the boyfriend eyed it, and we decided to give it a go. We sat on the patio and had some delicious beef green curry (me) and their beef/rice special (him). I was expecting Bob Likes Thai Food or Urban Thai Bistro quality food, but it was certainly a few steps above, with some authentic ingredients (pickled galangal, pea eggplant, etc.) and beautiful presentation. It's definitely the best casual Thai food I've had in the Lower Mainland, complete with Thai condiments on the table - roast chili flakes, crushed peanuts, chili oil, etc., so you can make it as spicy as you want. It's the kind of place where you order at the counter, they bring it to your table, and you're expected to bus your own table after your meal. The kind of place where you pour your own water into paper cups. The kind of place where, once you get your food, nobody's going to check on you again. It really does feel like you're going to be served fast food Thai, but the food exceeds your expectations - it looks beautiful and tastes beautiful. I'd dine there again if I was in New Westminster, but I don't think we'd take the Skytrain from East Van out to New West just to dine there. This isn't knocking Longtail - the food is delicious, but it's a really casual dining experience, I don't think of it as a destination dining experience.
Maenam is more of a refined dining experience - elevated ingredients, well balanced/sophisticated cocktails, proper table service, a contemporary/hip dining room. I remember dining on a halibut green curry the last time I was there and loved it. Maenam is to Thai food what Vij's is to Indian food, in my mind. I am no Thai food expert, however. If you had to choose one, Maenam would be obvious to me simply because you get the full experience, and it's a destination restaurant.