in very general terms, you'd expect an EL34 to be lighter in the bass end, more mid-focused and with crunchy high-mids and top end compared to a 6L6. the 6L6 will have more bass end, less mids, a more precise top end and will not crunch up as early (more headroom), plus they have more balance across the bass/mids/treble than EL34s.
if you can fit them in your amp, you might try some KT66s - they should bias up ok on the 6L6 setting (from memory). KT66s have all the best bits of both EL34s and 6L6s, imo.