it's not that marbles and black sequencer make each other superfluous - they just take up too much space together in this size case!! I've got both and a other sequencers besides (sinfonion, step fader, peaks, beatstep pro, pico seq plus some I'm probably forgetting - ah yes stages!) but I have much more rackspace - one or the other in this size case - personally I'd go black sequencer - if you want random - hit magic!!! and I truly love my Marbles
I know all about stages - I've had one since the 2nd batch - it's a great module - but it's also good to have different modulation sources - I usually recommend maths as a first modulation source - mostly because of the fantastic 'maths illustrated supplement' - which is a great introduction and jumping off point for patch programming - an incredibly powerful modular synthesis technique... working through and building on the examples given in the supplement gives a great introduction to modular synthesis - rather than just patching some modules together (what I call 'synthesis with modules' - as opposed to 'modular synthesis')
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities