Core difference
MLGS advertises a recurring lead supply. Solo ads are usually purchased as traffic from an established email audience, with delivery and targeting depending on the vendor.
Cost structure
MLGS uses a recurring membership model. Solo ads are generally purchased by campaign or click volume, so the spending pattern is different.
Control
With solo ads, vendor selection, audience fit, tracking and traffic destination become major variables. With MLGS, the supplied audience is a core part of the service.
What to measure
For either route, measure cost per engaged lead, click-through, opt-ins, sales and revenue rather than relying on raw lead or click volume.