An external tuner is better, because it has wider range and less loss.  You 
should make sure that the internal tuner is not being used at the same time. 
I use the mfj989 with the tw1 talking meter.  I used a voltage controled 
oscellator before I had the tw1.
The virtually infinnate range of the tuner is nice if you need it.  So your 
question brings up many other questions.
1.  Are you running power?
2. What is the antenna or antennas being used?
3. On what bands are you using your desired antennas?
4.  Will the internal tuner tune your antennas?

I need the 989 for the 600 watts PEP output from the valiant, not for 
anything the 480 can push at it.
I am also matching a 40 meter double extended zepp from 6 through 160 
meters.  I tune it as a marconi on 160 against ground for best results.