simple..

if u like touch screen. just go for 5800XM
it is the first phone with symbian OS 9.4 @ S60v5
new ui,new interface...,big screen, good speaker...
but.. its a bit difficult to texting.. and the screen will vibrate when touching[some cases],,
bad camera[not so good]
overall it is phone that target to music and touch screen lover..
if u like better phone but isnt a touch screen lover...
just go for N79, symbian OS 9.3 @ S60v3 FP2
new ui,new interface[compare to FP1],good speaker,
good camera,has xpress-on-cover[can change the back cover and the theme will automatically change when the cover change]
the cons.. is the phone has smaller screen[2.4 inches,compare to 5800XM,3.2inches]

i sugest u to go for N79