Question on buying tickets via CrossCountryTrains website

If 4 of you want to sit together round a table only two of you can have window seats.

Would it be best to not select a seat preference at all?

this depends on which train and date, if you left it too late for a very busy train you could be scattered whatever happens. But is more likely that the system is issuing reservations to manage the train but in reality there will be spare seats and you can rearrange yourself when onboard. Either don't select a preference or select no preference if it's being pedantic! With Advance tickets you only have to be on the right train - you can swap seats. With Off Peak or Anytime you can even flex the train!

We are buying 3 months in advance, so I will choose no seating preference. Thank you

If your entire journey is on Cross Country Trains, then it should be OK to book ahead for the best deal. However, buyer beware as if journey involves another train company, you may not get the best deal. For example Bristol to York would be entirely on Cross Country but from nearby Bath is not.

If you say what journey you are taking and the date and expected price, then someone should come back and tell you if it is a good deal or not and if not, how to get the price down.

once you've got the seat numbers you can have a try at finding where they are. There are a variety of seating plans here, but not as simple as it should be.
https://www.crosscountrytrains.co.uk/travel-information/on-board/about-our-trains

first you have to read the notes to see which type of train operates the route you are using. Don't get too excited about the refurbishment, not many have been done. If you have seats in coach D there is a problem, there are two versions and I doubt they can guarantee which you will get.

Good to know about other train companies being more expensive. Are you aware of any expensive train companies with the following journeys?:
1) Oxford -> York
2) York -> Shrewsbury
3) Shrewsbury -> Oxford
4) Oxford -> Bath
5) Bath -> London

actually crosscountry is one of the more expensive. The real problem is that all the operators offer discounted advance booking tickts on their own routes but often fail to link up with others where a change of train is needed, so if you ask for a through ticket you will only be quote dthe more expensive flexible price. Generally all operators with online ticket sales sell tickets for all others at the set prices, you just have to be wary about the connections. Use NationalRail.co.uk to check shedules, it will offer the best ticket prices and now shows some tricks of splitting tickets, but does not sell anything - since you now have a crosscountry account just use that once you know the best plan! NationalRail is also where you check for any ongoing disruption.

Oxford -York is possible on a direct crosscountry train but check whether it's cheapet to split the ticket at Birmingham

Shrewsbury-York is a mix of Transport for Wales and change to Transpennine Express at Manchester.

Shrewsbury-Oxford is two legs, Shrewsbury Birmingham is little more than a commuter with trains shared by TfW and West Midands, then Birmingham-Oxford is crosscountry

Oxford-Bath (direct) and Bath-London are GWR.

Much of this confusion will be softened soon when Great British Railways is full up and running. But somebody has managed to archive the transition website explaning it all!!!

actually crosscountry is one of the more expensive. The real problem is that all the operators offer discounted advance booking tickts on their own routes but often fail to link up with others where a change of train is needed, so if you ask for a through ticket you will only be quote dthe more expensive flexible price.

That is a statement which is somewhat devoid of subtlety of facts. Cross Country often have very cheap Advance Fares. In fact looking at the Fares Manual the cheapest through Advance (train specific) fares from Shrewsbury to Oxford as an example are set by Cross Country as 'XC and Connections fares.
However another and more scenic route from Shrewsbury to Oxford is via/changing at Hereford. Although there isn't a through Advance Fare on that route it is actually sometimes cheaper than via Birmingham as a split fare- £9.20 Shrewsbury to Hereford hourly and as low as £13 Hereford to Oxford every 2 hours, depending how far ahead you book- on that £13 fare you also change at Worcester Foregate Street (in theory, but not always, same platform- the station only has 2 platforms).
Likewise the cheapest fare (but longer journey) from Bath to London is often to travel to London Waterloo (not Paddington) changing at Salisbury.
York to Shrewsbury- the cheapest route (but longer journey time) is often York to Leeds, Leeds to Chester (hourly through Northern train), Chester to Shrewsbury. If changing at Manchester you often have to transfer on the free bus from Manchester Victoria to Manchester Piccadilly, although there is an hourly train which calls at both Manchester Victoria and Manchester Piccadilly. At Piccadilly on that train you change from the remote Platform 13 into the main train shed.
Sometimes on the Manchester Piccadilly to Shrewsbury train the cheapest fare (surprisingly) is 1st class, not standard class- on the every 2 hours trains which have 1st class.

Depending on the makeup of the family you should look into buying either 2x Two Together Railcards, or a Family and Friends Railcard (£35) each to save 33% off all fares for travel after 0930 weekdays/anytime weekends

My advice for you as a visitor would be to just buy the family and friends or 2 together railcard and use any site to buy the tickets. Yes, there are ways to save money but it’s actually quite complicated to navigate unless you know the routes well.

I always buy my train tickets using the Uber app. It will show splits that save money and it’s fine to buy these. It’s a strange feature of the ticketing pricing structure that sometimes it’s cheaper to complete one journey with 2 tickets instead of 1.