There are a few things to consider:
Price
On average Thomson Reuters is known to be less costly than Bloomberg. One thing to consider when looking to save money is that most vendors will use some kind of ladder pricing. So if you cannot get rid of either Bloomberg or Thomson Reuters completely then you may not save as much as you expected.
Technology
Thomson Reuters is really big on feeds while Bloomberg is not. Bloomberg's main business is selling terminals with very little footprint in the feeds business, afaik. If you are happy that your-terminal-app-is-your-feed then this will not matter to you. To be fair Bloomberg does sell something called BPipe which is the competitor to the Thomson Reuters consolidated feed, known as Elektron Real Time, previously known as RDF. Most banks I know will have a Thomson Reuters feed, very few banks will have a BPipe.
If you are building something for yourself or you are not too concerned with latency and not worried about not receiving every tick then all of this may be irrelevant to you. If- on the other hand - you are building something that will be used by several users in your organization and you do not want a solution that requires each user to be a subscriber to the vendor's terminal application, then you'll need a true feed solution.
From my experience most small shops will be happy with the infra that the vendor's terminal application gives them. There are lots of in-house solutions out there that's been build around the vendor's terminal application as opposed to around a true feed. As long as you realize what that means I don't think this is a problem per se.
Quality and accuracy of prices
As far as stuff that's traded on a market place (i.e. an ECN or an exchange) then there's no difference between what you get from Bloomberg and Reuters except that they may have conflated the stream for you so that your are essentially not guaranteed to receive every tick. Again a true feed solution is better than a solution where you take the price off a terminal infrastructure.
For stuff that's essentially OTC you should act with more caution. Bloomberg is really big on fixed income instruments which means that sometimes they sit on pricing data that's just not available from anyone else simply because the data is 'born' in the Bloomberg network. I think this is getting less so over time as fixed income liquidity these days does tend to get more concentrated on ECNs rather than on one-to-one closed environments (i.e. old style OTC). It all depends on which types of bonds we are talking about.
ISIN download information
By this I suppose you mean information that will allow you to map from an ISIN code to the vendor's instrument naming scheme (known as BSyms/BBGIDs for Bloomberg and RICs for Reuters) ? Firstly I hope you realize that this is a one-to-many mapping, not a one-to-one mapping. ISINs represent a unique identification of a financial (securitized) instrument while BSyms and RICs represent a financial instrument on a given 'market'.
Both Bloomberg and Thomson Reuters provide services that will allow you get hold of such mapping information. Of course such a feature is available in their respective terminal applications (i.e. BB Terminal and TR Eikon) but I'm assuming you are asking for such a service outside of the terminal realm ? Both vendors provide such service independent of their terminal apps but they will make you pay for it, afaik.
Other alternatives
There are a few other vendors out there of what is known as consolidated feeds. Probably fair to say that of those vendors with a global reach Interactive is #3 in this space. There are regional vendors as well, e.g. vendors specializing in the North American market only, etc. These alternative vendors very much so compete on price with the "the big two", i.e. Bloomberg and TR, so you can get a good deal with them. From my experience the cost reductions are however minimal because many organizations will not be able to get totally rid of "the big two" and then you suddenly end up with 3 solutions in your org, rather than one or two. YMMV.
