If you are talking plinking and training, either would be fine. However, if you are thinking that either would be a dependable combat load, I wouldn't put that much confidence in it. Here's a good post on the 5.56 M193/M855 by Dr. Gary Roberts:
"As I recently stated in my presentation at the NDIA in Dallas this past week, 5.56 mm NATO 62 gr SS-109/M855 FMJ was designed over 30 years ago as linked machine gun ammunition to be fired from the FN Minimi/M249 SAW while engaging enemy troops wearing light body armor during conventional infantry combat at distances of several hundred meters--while not a perfect solution, M855 does perform adequately in this role.
Unfortunately, combat operations since late 2001 have again highlighted terminal performance problems, generally manifested as failures to rapidly incapacitate opponents, during combat engagements when M855 62 gr “Green Tip” FMJ is fired from 5.56 mm rifles and carbines. This is not surprising, since M855 was not originally intended for use in carbines or rifles, especially those with short barrels. In addition, most current issue 5.56 mm bullets are generally less effective when intermediate barriers, such as walls, glass, and vehicles shield opponents--this is a significant consideration in urban combat.
As an interim solution to these problems, deployed SOF units have used 5.56 mm Mk262. The Black Hills produced Mk262 uses the 77 gr Sierra Match King (SMK) OTM and is built as premium quality ammunition intended for precise long-range semi-auto rifle shots from the Mk12 rifle. It is great for its intended purpose. Mk262 has demonstrated improved accuracy, greater effective range, and more consistent performance at all distances compared to M855 when fired from current M16, Mk12, M4, HK416, and Mk18 rifles and carbines. However, despite this substantially improved performance, Mk262 still manifests the problems of poor intermediate barrier penetration and somewhat variable terminal performance inherent with the SMK design.
The disturbing failure of 5.56 mm to consistently offer adequate incapacitation has been known for nearly 15 years. Dr. Fackler’s seminal work at the Letterman Army Institute of Research Wound Ballistic Laboratory during the 1980’s illuminated the yaw and fragmentation mechanism by which 5.56 mm FMJ bullets create wounds in tissue. If 5.56 mm bullets fail to upset (yaw, fragment, or deform) within tissue, the results are relatively insignificant wounds, similar to those produced by .22 LR--this is true for ALL 5.56 mm bullets, including military FMJ , OTM, and AP, as well as JHP and JSP designs used in LE. This failure of 5.56 mm bullets to upset can be caused by reduced impact velocities when hitting targets at longer ranges, as well as by the decreased muzzle velocity when using short barrel carbines. Failure to upset can also occur when bullets pass through minimal tissue, such as a limb or the torso of a thin, small statured individual, as the bullet may exit the body before it has a chance to upset. Finally, bullet design and construction plays a major role in reliable bullet upset. Without consistent bullet upset, wounding effects are decreased, rapid incapacitation is unlikely, and enemy combatants may continue to pose a threat to friendly forces and innocent civilians.
Angle-of-Attack (AOA) variations between different projectiles, even within the same lot of ammo, as well as Fleet Yaw variations between different rifles, were recently elucidated by the JSWB-IPT. These yaw issues were most noticeable at close ranges and were more prevalent with certain calibers and bullet styles—the most susceptible being 5.56 mm FMJ ammunition like M855 and M193. What this means is that two shooters firing the same lot of M855 from their M4’s with identical shot placement can have dramatically different terminal performance results: one shooter states that his M855 is working great and is effective at dropping bad guys, while the other complains his opponents are not being incapacitated because M855 is zipping right through the targets without upsetting. Both shooters are telling the truth…"
Dr. Fackler also worked on a number of Vietnam M193 wounds up until the M855 was adopted and there was a good number of them when fired from a full length M16 20" failed to upset and cause fragmentation. So, the resulting wound was a little keyhole that didn't do a whole lot of wounding. The 5.56 is sporatic with fleet yaw and is MUCH better with soft points, OTM loads, or hollow points.
Here's a link to more on the subject:
http://www.dtic.mil/ndia/2008Intl/Roberts.pdf
There are a lot of cheap soft point, OTM, and hollow point 5.56 loads out there that would be better for defensive use than the M193 and M855. I suppose that if I had to use one for defense, I would choose the M193 though.
i shoot long range better.
