As someone with both MNK and SAM to 75, here's my thoughts.
MNK/DNC is an extremely strong soloer for most anything T and lower. First off, the majority of a MNKs damage is via the TP phase, not WSes. Also if you own Faith Baghs, then MNK/DNC becomes much more powerful. You can even use it for exp, in those situations pre-74 where you're asked to sub NIN. A NIN sub brings nothing offensively to the table for MNK, and neither does a SAM sub (HTH is not 1 or 2 handed. It is it's own category). So pre-74 where all you'd have is Ichi, /DNC would be of more use to the party.
/DNC for MNK has two main drawbacks. First, you can't really countertank like on MNK/NIN. Counterstance is usually a no-no /DNC, when you get hit it hurts a LOT more (since you usually aren't targetting mobs that cap Att against you when solo). Secondly, on stronger mobs /NIN is required to solo, due to how hard they can hit.
SAM/DNC is an absolute beast of a soloer, but it requires getting your hands on a Soboro. If you don't have Soboro, it's just a so-so job combo. Seigan + TE plus Drain Samba II + Healing Waltz II make SAM/DNC, swinging a 450 delay G Katana which attacks 1-3 times, a very tough job to kill while solo. The HP regenerative capabilities, plus goodies like meditate and Sekkanoki, are golden. Again like with MNK, if mobs hit really hard, you aren't soloing them.
WHAT SOLOS/FARMS BETTER?? MNK/DNC, SAM/DNC or NIN/DNC at 75??
Solo farm? Depends on the target. But for low level farming, THF/XXX or XXX/THF. MNK/THF works great, as Chakra will heal the minor damage that you take over time. High level stuff? NIN/DNC if it can be blink tanked, or RDM/NIN. Your question is too open ended.
RNG:75 MNK:75 WHM:75 BRD:75 BST:75 SAM:75 WAR:75 THF:75 BLM:75
DRG:72 SMN:63 DRK:55 NIN:49 PLD:42 RDM:41 DNC:37 SCH:37 BLU:37 COR:20 PUP:22
Woodworking:88 Cooking:60 Alchemy:60 Bone:60 Leather:60 Cloth:60 Smithing:60 Gold:54 Fishing:33