On your 2 questions:
1) Space on Tape: if your tape device does compression on the fly as ours does. Our usual space saving is approx 65% when going onto tape.
This means that the 3.5GB file is stored on tape as 1.2GB - this is about 200MB more than the SQL 2008 Compressed backup.
If the original 3.5GB file is zipped using maximum compression then the ZIP file is approx 820MB (note: this maximum compression takes longer than the SQL 2008 Compression Backup)
So, this means that there is potentially 200MB that could be saved when storing onto tape for this 3.5GB database backup.
2) Reliability: because the SQL engine is doing the compression and not a third-party vendor, I am reasonably confident that this backup should be seen as reliable as the original .BAK without compression.