HARP is my choice for one shoots adventures since the character creation is streamlined for speed and creating low level characters that are competent.
It sure works out great for low level character. The campaign I GM I started them out at level 0 and they are now around level 10. In HARP characters advance a lot faster, so level 3-4 character can pack a good punch. And even at level 1 they are not hopeless.
RMFRP is my choice for long campaigns since it handle the epic scale better. The downside with this is that RMFRP characters need to be higher level before they actually reach the wanted archtype.
I agree. The problem is that at low level RM characters are weak. 10-15 years ago when I played RM2 on a regular basis a normal campaign started with characters around 6th, 7th level.
Conversion between the to are easily enough. If you prepare monsters and encounters before playing it should be no problem at all, and on the fly conversions can be done accurately enough with good knowledge of just the one system you play as long as you have familiarized yourself with the other system.
I have yet to try to convert player characters, but as I feel I'm getting closer to a more epic level of play I am considering changing to a variety of RM. Probably RMC since it's the closest to good old RM2 which was my main RPG during my most active period as a player end GM.
As with most RPGs I have played, GMed or just read fighters tend to fall loose to spell users in the long run, but there is always the option of letting your fighters learn some magic when they feel they have mastered the art of banging people on the noggin'