"These two if my safari were for game that is typically on the lighter side (Wildebeest sized):"
Wildebeest- lighter side- Huh?- not the same wildebeest I'm familiar with!
For all plains game... My idea of "lighter" is slightly different- more like: impala, gazelle, springbok, bushbuck, reedbuck, etc. Wildebeest are NOT small (light), with the blue variety males at about 550 lbs and they have a well earned reputation (and by my experience) for being one of the tougher plains game as do the oryx and zebra. While southern greater kudu are slightly larger than blue wildebeest they do not have a reputation for being especially tough (also by my experience).
My arbitrary, subjective categories of size could be broken into four groups: 1) large- eland, a large eland bull really outclasses all other plains game for size, 2) medium- oryx, wildebeest, hartebeest, waterbuck, sable, zebra, kudu, etc., 3) light, small: impala, gazelle, reedbuck, bushbuck, springbok, etc., 4) very light, very small: duiker, klipspringer, grysbok, oribi, steenbok, etc.
Of course the 6.5 x 55 will kill plains game. Would I choose it on purpose for large plains game like eland- no. Too many other, better choices.