I personally prefer Rondon, there is more variety in the goals he scores and they are all in open play as he doesn't take penalties for Zenit. Bacca's stats for goals and assists are great, but like Balotelli and Lambert before him the stats a skewed slightly by goals from penalties.
Personally I couldn't care how the goals are scored as long as they are scored.
Bacca[2012/13]
Jup. Pro League: 25 games, 22 goals, 4 assists
Europa Lge: 7 games, 3 goals, 1 assist
[2013/14]
La Liga: 35 games, 14 goals (13 without pens), 9 assists
Europa Lge: 15 games, 6 goals, 3 assists
[2014/15]
La Liga: 37 games, 20 goals (14 without pens), 7 assists
Europa Lge: 15 games, 7 goals, 3 assists
Rondon[2012/13]
Premier Liga: 25 games, 7 goals, 1 assist
Europa Lge: 12 games, 6 goals, 1 assist
[2013/14]
Premier Liga: 20 games, 13 goals, 4 assists
Europa Lge: 4 games, 4 goals, 0 assists
[2014/15]
Premier Liga: 26 games, 13 games, 3 assists,
Europa Lge:6 games, 3 goals, 0 assists
Even without penalties Bacca scores and assists more.
You say Brendan likes his strikers to get involved, going off goals and assists, Bacca gets the most involved. Passing percentage is one of the worst stats to judge a strikers involvement on.