Whilst it may be possible to do 6-8 milk based coffees in a row with a Levetta (pouring all the shots then steaming the milk), its not ideal. And, being a single boiler machine, switching back and forth from brew temp to steam temp is also not ideal (it would require time to settle back down to brew temp after steaming).
So, whilst the Levetta may be twice as good (in build quality, group head, large boiler, etc) than the 6910, the Sunbeam would have the advantage in being able to do multiple milk based drinks, having a separate thermoblock dedicated to steaming (if you dont mind the potential problems with quality of build and components). In this capacity, the 6910 is pretty much unmatched at its price.
Otherwise, if you want a good quality machine and if being able to make this volume of milk based coffees on a regular basis is important to you, you may need to consider a HX machine ($1500 plus). Also keep in mind that a quick, good quality grinder would also be preferable in doing this amount of coffee.
I recently had a week of making multiple coffees with my Lelit combi (having taken it away on a holiday). Found it difficult and frustrating due to the relatively slow and doserless (Lux) grinder and limitations of the single boiler. Im lucky that doesnt happen too often, and it does a good job for 2-3 at a time, but Im thinking of a HX down the track.
Just my 2 cents worth.