Do you mean between the two machines or between the two O/S's?
Between the two machines, the obvious thing straight away is that the N54l is twice the size of the EX495 due to the optical drive and overall design, truth be told the N54l seems like a lower quality piece of kit (understandable considering the price). Its only advantages over the EX495 are the ease of memory upgrade (and cheapness of DDR3 vs DDR2), on-board RAID, and VGA port (retrofit-able to the EX anyway as it has a header on the board).
By comparison the EX has higher build quality (vibration is less noticeable), a slightly more powerful CPU, tool-less drive caddys and smaller form factor. The disadvantage though is unless you want to retrofit a VGA port and shell out for a big stick of DDR2 (one ram slot) WHS V1/'11 are your only options.
Between the two O/S's (WHS v1 & S2012 Essentials), well for a start one is based on Server 2003 and the other on Server 2012, so the is an obvious difference there (by comparison WHS 2011 is based on Server 2012 R2) as the are a lot of advancements behind the scene. Differences between the O/S's depend what you want to do with them, connecting via RDP works the same*, running a torrent application for acquiring linux .iso's is the same, shared folders and DNLA media streaming works just as well. The big difference though is that while WHS 2011 dropped drive extender (for many the best feature of WHS and the reason 2011 uptake was so poor) the is a new and improved version in 2012/W8 called storage spaces which works great.
Overall I think if you can get S12k Essentials free via Technet or something I would say defiantly do it, but if not your better off saving the pennies and going with WHS v1.
*Something to note is that using the W8/2012 remote desktop app to connect to another W8/2012 machine is flawed as hell as moving the mouse to a corner brings up the local machines effect not the remote machines (it all works fine using the normal RDP program though).
