This is coefficient of variation (SD over Mean). A dimensionless index.
To answer your question, i'll take a reliable player with average 35 over unreliable player with avg 47. Reason is simple as reliable player would contribute toward team
lot more often than unreliable player.
With reliable players of avg of 35, team would score 550+ per test (enough to save, draw or win). With unreliable players, you may score 700+ or 400- and that will hurt you a lot. (Pakistan team is an ideal example of this phenomenon called "collapse")
An example (extreme to convey the inherent flaws on arithmetic mean):
3 test series:
Batsman A: 0,0,300,0,0,0 Avg: 50
Batsman B: 35,35,35,35,35,35 Avg: 35
So a player with a high average with low reliability will actually hurt team more than he would benefit it.
To make it more universal, we can start with following value:
http://stats.cricinfo.com/ci/engine...=team_average;template=results;type=aggregate
Avg: ~32 per wicket and ~979 runs per match. ~245 per innings.
so we can safely say that cut off value is around 32 +-3
so reliable player with avg of 35 is good enough to produce satisfactory results for team.
In-case you have a player with higher average and also with great reliability then we must choose that player.
Otherwise if average is above ~35 than I'll go with a player with higher reliability. (Though this also depends how much difference they have between their reliabilities)
i.e. I'll pick Wasim Raja over Zaheer Abbas (the Asian "Bread"man)
Hope I am clear enough.