I think it depends on the type of game you are playing and your personal style of play:
Proxies: I usually prefer proxies over RTMs because you can "set it n' forget it," just like the popular As Seen on TV slogan says. I play a lot of conflict-style matches and typically use a very aggressive "run n' gun" strategy. A big part of that game-play style, I think, is the ability to control key corridors and areas while you move through the map. It essentially allows you to pick up kills while being elsewhere, maximizing your effective span of damage. Although some regard it as cheap, you can also mine spawn points that you can't directly see. Defensively, you can use proxies to "watch your back" and keep enemies from approaching on your blind side. I don't usually snipe, but obviously, this would also be useful. Not having to lower your weapon to trigger them with your phone is a big advantage. The downside is that proxies take a few seconds to arm, and by setting them, can leave you momentarily open to attack. As a general rule, if you see someone setting a proxy, it is usually an absolute green light to attack immediately. As with all mines in GE, they disappear when the person who set them is killed.
RTMs: Remote trigger mines, although not usually my preference, can also be a VERY formidable weapon. These are great for setting traps, such as triggering/shooting them as someone walks through a door. Perhaps the best use of RTMs are as "exploding Frisbees," throwing them at enemies and then triggering almost immediately. In team games, this can be a definite advantage over proxies. Because RTMs detonate when triggered, there is no arming time, and can more easily allow you to get multi-kills. Throwing these into a group of enemies is the most common way in GE to get the elusive "quad kill." The downside to these is that you usually need line of sight to use them effectively (except in Goldeneye mode when placed on consoles). Also, as stated above, detonating these requires you to drop your weapon and pull out your cell phone, which can leave you momentarily open to attack.
Overall, I prefer proxies over RTMs in most situations. It should also be noted that you can shorten the fuse time on frag grenades to achieve a similar effect to the "exploding Frisbee" as well. Equipping either type of mine still gives you grenades.