I'm not sure if the default mail client keeps the mail on the SD card, it does keep the attachments. So I say Gmail, given it does confirm it with an option.
I hate using internal storage.
The default has, Gmail app doesn't:
Copy & Paste
Better look (Sense UI theme)
Attachment view
New Mail preview in notifications (barely)
^This upsets me becuase I want the best of both worlds lol
I set my college email (Google Apps) & corporate mail to the default app.