m.w. freebase 263.2
m.w. hcl salt 299.8
HCl - freebase conversion ratio 0.78 (1mg tramadol HCl = 0.78mg tramadol base OR 1mg tram base = 1.28mg tram hcl)
Base is substantially less soluble than the HCl in water.
Base is probably lower melt/boiling points than HCl. (not that i would smoke it)
base is ... basic. (high ph) tramadol hcl is normally slightly acidic (low ph)
if you take it orally, there is no difference (hcl in stomach converts it to hcl salt)
you probably won't 'be able to dissolve it in plain water for injection, plugging etc... unless you add 1 mol eq of your fav acid (hcl, citric, acetic).. even then check ph before injecting or you can fuck your veins.
low solubility means it wont sublingual/nasal absorb as well as hcl salt
most people would want to take tram orally anyway (more conversion to desmethyl-tramadol via liver) so there is no difference aside from being 30% more potent by weight