M16A1 is more reliable in my opinion. Later on you can try and grind using the Thompson with the drum mag. Also, typically DMRs aren’t the best choice when it comes to grinding. You’re better off with automatics or explosives.
Most times, don't ask which weapon is better. It entirely depends on your play style, and what weapons you're good at using. With a Dragonuv versus a Wingmaster, those are two completely different weapons. One's a shotgun, one's a sniper. What are you better with?