Series 9 - is the latest version.
Smaller number eg Series 7 or 5, is the older version.
Buying a pc whether laptop or desktop, etc. you need to understand certain things.
-What is your need and/or purpose of getting the new pc for.
-Budget, ignore this if it is not an issue to you
-Spec. CPU speed; >3.0 Ghz CPU dual core; quad core, RAM= 8MB at least; etc. and video graphic
-Fixed Additional. Include DVD-Writer, USB 3.0 (how many you need minimum - for printer, mouse, additional Hard disk [to watch movies which should be store in External HDD ), Wireless (speed g/n), hard disk storage - 500GB to 1TB. External Monitor connector (so that you can connect to, say 42" TV with monitor connection - to watch movie) **Fixed Additional - so that you do not carry or add more device and carry along = weight.
-Bag: Get a good bag with enough space and protection to the LED or LCD. To protect your investment.
-Battery. This is very important if you use it outside, most of the time. If plenty of money - get a spare battery.
-Warranty. Make sure it come with International Warranty card. Note. Buy from distributor or well-known company (being in retail for many years, NOT just new company). Keep your receipt.
-Handover. When you just paid, let their sales staff to open and check for any damages, scratch (especially the LCD), etc, in front of them. When you are home, run the pc - let it on for a many hours. If there is fault, it will show. [it did happen to me, and they get another new on to replace, on the spot. Follow the same procedure, and make sure what you see them put back into the box and you immediately take over. This is to avoid them to change to another one. Believe me, i saw this happened before.]
**Get everything you need (think) so that to avoid going another shop to get accessories. If the accessories was not bought from them, any problem you have, they will blame the accessories of other shop.
Tested out both, I feel the keyboard on the series 9 is not that 'hot' as the series 7. I'm pretty unsure now, maybe i'll just take my money and ask the guy in the shop many questions about what is best for me.
my best advice is to get the best bang for your buck. choose your budget than find the best stats you can and the best reviewed laptop.
personally i have/had two HP laptops both of them ultrathins and havn't paid more than 700 US dollars for them. my current one has a AMD A6 2.10 ghz processor, 6 gigs of ram, 250gb ssd and gets close to 10 hrs battery life if i'm on moderate usage. just food for thought