Upgrade to OS X 10.10 from 10.6.8

Question

I would be grateful for advice about upgrading my OS.
I have got a MacBook Pro bought in September 2013 in the UK which is running under OS X 10.6.8, with 2.26 GHz Intel Core 2 Duo, 4GB of memory, and a 500GB hard drive, mostly unused. Some software I've just bought needs OS X 10.7 or later so I intend to upgrade.
Looking on the web, I think my machine can support OS X 10.10, and I gather that it's a simple, though lengthy process, to upgrade from the App Store. Since I'm starting from OS X 10.6.8 I understand it's free.
The important non-Apple applications I'm running are MS Office for Mac and Adobe Acrobat and Photoshop.
I'm running Time Machine to an external hard drive, so I understand that if anything goes wrong I can restore my system and data to what they were before.
Should I go to OS X 10.9 or earlier rather than 10.10? I'm unlikely to make much use of whizzy new features; performance is more important to me.
What can go wrong? What precautions should I take?
Seamus O'Connell

Answer 1

Many thanks to Eric, dwb, and Jimmy for all this information. I’m still digesting it, and the further information in the links within the links. I have made significant progress.
Right now I would be grateful for thoughts on further questions, in bold in the notes below. It's quite likely that these notes go into more detail than you want to read. If so, please just look at the specific questions.
GENERAL PERFORMANCE
I replaced my 4GB of RAM by 8GB from Crucial (arrived next day). It made a great difference. I’m now considering replacing my hard drive by an SSD. That would be tied in with a new strategy for backups, below, in which I could use the HD for a clone backup.
What might I have got wrong with the Apple Hardware Test? I held down D while restarting and while powering on but nothing special happened. The machine started up normally.
I uninstalled Carbonite. It kept getting the colour wheel in the menu bar and in System Preferences and used a lot of CPU even though it said it was idle. I might reinstall it if its problem with Yosemite gets sorted out – my subscription runs until August.
I had problems with Dropbox. They now seem to be cleared, not quite sure why. Seem to have to run it every so often to get it into the menu bar, then it disappears again.
SECURITY
Any suggestions for an anti-virus product on a flash drive which I could run from time to time? JimmyCMPIT suggests this. I got rid of Sophos, but I would like to be able to check for malware which affects Window machines – I also use a PC.  ClamXav get lots of mentions but several reviews don’t think much of it.
I installed the Firefox add-on NoScript. I shall do my banking using Firefox with NoScript from a user account (which I would anyway), on both Mac and PC. NoScript is freeware but if it turns out to be as good as claimed I shall make a donation.
I cleared the download folders in all my accounts and will keep them empty.
I went through the settings in all browsers in all accounts – auto-update, checking options allowed, add-ons and plugins. And auto-update for OS and MS Office. On both Mac and PC I have four user accounts and an administrator account. On the Mac I have Safari and Firefox, on the PC IE, Firefox, and Chrome.
BACKUP AND RECOVERY
Two or three years ago my hard drive failed. I had to rebuild everything from scratch. I thought that since I had Carbonite it would not be much problem – I did not then have Time Machine. In fact it was quite difficult and took a very long time, even with very effective assistance from Carbonite support staff.
A bootable clone would save a lot of time. It seems to me that I would make a clone backup just before a major upgrade, in case the upgrade turned out to be unviable. Once the upgrade showed that it was stable I would overwrite the pre-upgrade backup by a post-upgrade backup to fall back to subsequently if necessary. After that I would every so often replace it by a more up-to-date backup. I’d keep it off-site.
Carbon Copy Cloner seems to be the popular choice.
If I bought an SSD drive could I put the current internal drive into an enclosure and use it for clone backups? Alternatively I could use an old external drive I’ve got.
What sort of SSD should I buy? The Crucial website lists several for my machine (MacBook Pro 13” mid-2009). The cheaper ones have slower write speeds, from 150 MB/s to 370 Mb/s. How significant would that be?
Will I have to deal with TRIM?
Is it possible to recover from the bootable clone then tell Time Machine to restore all user files created/changed since the date the clone was created?
One of Eric’s links suggests also making occasional off-site copies of important user data on non-rewriteable media, i.e. DVD-R. I could get everything important onto four disks.
I would set up a similar backup strategy for my PC, with Windows 7 built-in back-up and Carbonite taking the place of Time Machine.
Does the strategy below look sensible?
Mac:
Internal hard drive: SSD
Bootable clone: HD - before then after major upgrades
On-site contemporaneous user data backup: Time Machine on external HD
Off-site contemporaneous user data backup: None, unless Carbonite gets fixed
Non-rewriteable user data backups: 4 x DVD-R - say two sets cycling monthly, kept off-site
PC:
Internal hard drive: HD
Bootable clone: HD - before then after major upgrades
On-site daily user data backup: Windows 7 backup on external HD
Off-site contemporaneous user data backup: Carbonite
Non-rewriteable user data backups: 4 x DVD-R - say two sets cycling monthly, kept off-site