The key difference between the two accounts is that an HRA is employer-owned whereas an HSA is employee-owned. This means an HRA is left behind when an employee no longer works for that employer, and the funds are no longer accessible.
In general, HRAs have no “use-it-or-lose it” policy. The employer can specify at the beginning of the year whether funds remaining in a participant’s HRA are either forfeited to the employer at the end of the Contract Year or whether funds can roll over and remain in the account from year to year. This means that you need to know what your employer's policy is regarding HRA funds. Does your employer have a "use it or lose it" policy?
In general, HSAs have no "use it or lose it" policy. However, a similar plan, FSA is "use it or lose it". These types of accounts can be established for more than healthcare, but may also cover childcare, elderly care, orthodontia, Lasik, etc.
Basically, if you have healthcare expenses that need to be paid for 2018 and have an HSA, HRA, or FSA account, now is the time to finish those transactions. The risk of "use it or lose it" is enough for me to sit down and pay my medical bills before the end of the year. I'm not sure when your employers' list their cut off dates for reimbursement, but I don't want to lose anything!