Call Function For Inner Join

I have a procedure which has query
like Query 1.

Query 1

Select Clinetid
from clinet
inner join {


select centerid from GetChildCenter(@Centerid)
union
select centerid from getParentCenter(@Centerid)
} as Center c

on c.Centerid = client.Centerid


Query 2

declare @Center table ( centerid int)
insert into @Center

select centerid from getchildCenter(@Centerid) union all select centerid from getparentcenter(@Centerid)

Select Clinetid
from clinet
inner join @Center c on c.Centerid = client.Centerid





I just want to know which one is better performance wise..
because there is millions of rows for table center which is used by function getChildCenter() and GetparentCenter()