Mobility vs Equipment???

I just began purchasing lighting equipment and I'm not sure what direction I should go in.  I have one ab800 light right now and I know I need to get some modifiers.  Looking at the soft sliver PLM.  My question is, to stay within my budget for right now would it be more beneficial to purchase the battery back for the lights to be able to shoot wherever, or to get an additional ab800 and be restricted to shooting where I have access to an outlet?

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