Gerrard is a world class player he is versatile as he can play in many advanced attacking midfield positions and is a player who has an eye for a long and short pass (got 104 assists for Liverpool), has running and dribbling skills also he can shoot from long range and has ability to score goals (got 151 goals for Liverpool).
Click the link to see Gerrard's career statistics see here:
http://soccernet.espn.go.com/player/_/id/7935/steven-gerrard?cc=5739
Steven Gerrard club honours:
Liverpool
FA Cup (2): 2000–01, 2005–06; Runner Up (1) 2011–12
League Cup (3): 2000–01, 2002–03, 2011–12; Runner Up (1) 2004–05
FA Community Shield (2): 2001, 2006
UEFA Champions League (1): 2004–05; Runner Up (1) 2006–07
UEFA Cup (1): 2000–01
UEFA Super Cup (2): 2001, 2005
Lampard is a different type of player but also world class he is is more of a hard working box to box midfielder who makes late runs into the box, he can't dictate the play but can keep possession and possesses good technical abilities also his long balls and final ball are exemplary (got 120 assists for Chelsea) and he has the ability to score lots of goals (got 189 goals for Chelsea).
Click the link to see Lampard's career statistics:
http://soccernet.espn.go.com/player/_/id/8941/frank-lampard?cc=5739#
Frank Lampard Chelsea club honours:
Chelsea
FA Premier League (3): 2004–05, 2005–06, 2009–10
FA Cup (4): 2006–07, 2008–09, 2009–10, 2011–12
Football League Cup (2): 2004–05, 2006–07
FA Community Shield (2): 2005, 2009
UEFA Champions League (1): 2011–12
If you can only have one of the two I would still select Lampard in my team a head of Gerrard for Lampard's goals and assists and have the team built around Lampard.