Spring; trigger response and consistency.
Electric - never going to be absolutely perfect trigger response (though you can get quite close if you spend enough). Also they're not as quiet most of the time (if this concerns you).
Gas - not consistent enough. Cooldown and variable weather is going to screw you over. The solution to this is rigging your mags for HPA but that's a whole different ballgame. There is a Czech(?) company that makes a drop-in bolt, but I cannot for the life of me remember their name or the URL. *Edit Link for this has been posted below by Shizbazki*
TM VSRs are widely regarded as being pretty much the best springer sniper rifle out of the box. However, the absolute best ones require patience and cash to get right. Expect to spend around £450 for absolutely everything. PDI and Laylax/Prometheus/First (their name changes spending on the day of the week) make the best sniper rifle bits. My opinion on sniper rifles is to go big or go home on the budget if you really want to compete, but I know a lot of people don't share that opinion so take it as you will.
Try X-Fire.org for PDI as some of their stuff just isn't stocked anywhere else. Laylax Zero Triggers and the like are easy to get in the UK.
What you're going to want:
New cylinder
New spring(s)
New nozzle
New guide rod
New hop up unit
New trigger unit
Barrel spacers
New barrel (go for 6.05 or wider)
Aftermarket cocking handle/charging handle/cocking lever (even the TM ones break)
A decent scope (an absolute must - doesn't need to magnify a ludicrous amount but it does need to be clear. Visionking make very good rifle scopes that can be had off DHGate for very good prices)
Decent-ish scope rings, but don't spend more than £25 unless you want the aesthetics, frankly
And finally your hop. I'd recommend a flat hop with R-Hop of some description to get maximum lift as you'll be wanting to use at least .30s (preferably as high as the mid .40 range).
There are some good threads already on here about a lot of this:
http://www.airsoft-forums.co.uk/index.php/topic/24745-upgrading-a-marui-vsr-10/
http://www.airsoft-forums.co.uk/index.php/topic/18557-help-on-sniper-rifles/
That was from a couple of minutes searching. If you can, I'd really recommend spending a good few minutes as literally every question about sniper rifles has already been answered on this forum (let alone other, dedicated sniping forums like http://www.airsoftsniperforum.com/)