I've done a ASE -> MS-SQL migration - in my experience 90% of the task is easy - but depends what code has been written.
If you have a large amount of data I'd recommend splitting the data migration out from the
code migration.
Copy the data - using whatever tools you have (bcp) and then set up replication to feed ASE->MS-SQL. Spend a bit of time here taking advantage of some of the MS-SQL features - eg indexes - some covering indexes as you can have unique indexes with included columns.
Then migrate the code separately - this will give you a complete parallel system to test with.
MS T-SQL really to be a superset of ASE T-SQL so the MS conversion tool works pretty well.
The advise from MS is to remove the performance hints (forceplan index, hints, etc). Its worth trying but for us the disadvantages out-weighed the advantages.
Check all the setting you can use - like ansi null setting - they can be handled differently. There are a few other.
Quick note on performance - and this is only my personal experience. Out of the box - MS-SQL performance is better than ASE - but ASE has much more configuration options and when tuned (again in my personal experience) ASE can perform better. So whatever ASE machine you have, purchase a slightly bigger machine for MS-SQL and you should be fine (it'll only cost a bit more anyway).
Hope this helps.