You will eventually have to upgrade tanks for most serras... unless it were a Sanchezi or something smaller. Exos will give you the most entertainment, as it can take years for smaller rhoms and serras to actually be somewhat active.
Gorwing out my rhom for over 12 years is something I enjoyed, but not sure I would do it again.
Gorwing out my rhom for over 12 years is something I enjoyed, but not sure I would do it again.