GIS Analysis vs Visualisation: Key to the discussion is a previous post of mine on the use of GIS in schools in the UK. In it I said that curriculums aren't asking specifically for GIS analysis, they're actually asking only for Visualisation. Actually the national curriculum does ask for school teachers to use GIS analysis in classes (see here link via Jason Sawle). I think this is wrong educationally, you can deliver meaningful school level Geography education by getting students to use GIS visualisation without using GIS analysis. It's an important distinction because GIS analysis is complex for school children to undertake whereas its much simpler to do GIS visualisation with Google Earth. I would love to see a teacher try and get the students to complete the activity they describe here in the allotted 20 minutes - more like 1.5 hours I reckon.
ESRI DigitalWorlds: (hereafter DW) I've spoken at length to Jason Sawle of ESRI who is one of a team of two to produce DigitalWorlds, a school GIS. In comparison with Google Earth some facts that aren't in dispute:
- Cost: DW is £250 per year, GEarth is free
- Lesson Plans: DW comes with a series of lesson plans, 24 in total with 7 more in development. To my knowledge there are 16 Google Earth lesson plans available on the web(1). I can't vouch for the quality of any of these (except my own)
- Home: GEarth can be used on home computers, DW licence doesn't allow this.
- DW features not in GEarth: comes with historical and modern OS maps of the UK, GIS like thematic/symbols mapping, graphing of points, a wider range of background imagery to choose from and better functionality to produce photos in pop ups.
- GEarth features not in DW: 3D visualisation, historical satellite imagery, time slider, Google Earth tours, all the content in layers.
What we disagree about:
- Feature Set: Which of  or  is more important educationally (BTW I may not have caught every difference)
- GIS in schools: I think 'simple is best' in this case and that GEarth wins on this count. Jason disagrees with me, he argues that DW has been developed collaboratively with teachers and stakeholders, that school students can handle GIS and that GIS enables 'real world' scenarios to be studied in class as GIS is what is mainly used in industry (I don't deny GIS is industry standard). He admitted that they hadn't done any explicit usability testing of DW which is my main problem with it and GIS generally in schools.
Conclusion: So to be fair to DW, if you think the lesson plans are worth it and you rate  over  you may want to buy the software. Also, the National Curriculum calls for explicit GIS analysis use which is to DW's benefit in comparison with GEarth. However, your specific exam board may interpret the use of GIS in a more relaxed manner meaning you can just use GIS visualisation and Google Earth (certainly the situation for EdExcel from my reading).
Jason made the point that it isn't necessarily an either/or situation, that you could use GEarth for some things and DW for others. I think he's got a good point from an educational viewpoint but in reality, few schools could afford to do this as the cost issue comes into play.
1] 11 at Juicy Geography, I've done 2 and Google have 4 (secondary Geography - Google also do primary ones which I haven't counted)