Either will perform the task at hand , you can turn pens with both and there are mandrels for both sizes and adapters for MT1 to MT2 . With that said though , the short answer is , yes MT1 can hold you back . Adapters are fine but they will introduce extra run out and shorten the bed length .
The smaller size of these mini lathes are lighter in weight and less rigid in design , they also have smaller motors . These factors will also reduce the usefulness of these lathes .
To start turning pens these lathes will be fine but be expecting to replace them sometime in the first year if you ever want to turn anything other then pens and small boxes and if you ever want to step up to doing things like drilling on the lathe , they just aren't designed for these tasks .