Postgresql vs SQLserver for this application ?

hi all.

We are designing a quite big application that requires a high-performance
database backend.
The rates we need to obtain are at least 5000 inserts per second and 15
selects per second for one connection. There should only be 3 or 4
simultaneous connections.
I think our main concern is to deal with the constant flow of data coming
from the inserts that must be available for selection as fast as possible.
(kind of real time access ...)

As a consequence, the database should rapidly increase up to more than one
hundred gigs. We still have to determine how and when we shoud backup old
data to prevent the application from a performance drop. We intend to
develop some kind of real-time partionning on our main table keep the
flows up.

At first, we were planning to use SQL Server as it has features that in my
opinion could help us a lot :
- replication
- clustering

Recently we started to study Postgresql as a solution for our project :
- it also has replication
- Postgis module can handle geographic datatypes (which would
facilitate our developments)
- We do have a strong knowledge on Postgresql administration (we
use it for production processes)
- it is free (!) and we could save money for hardware purchase.

Is SQL server clustering a real asset ? How reliable are Postgresql
replication tools ? Should I trust Postgresql performance for this kind
of needs ?

My question is a bit fuzzy but any advices are most welcome...
hardware,tuning or design tips as well :))

Thanks a lot.

Benjamin.